I'm the same weight as you, 51yo, 178cm and fairly conditioned, advanced although feeling my injuries and possibly getting held back nowadays. YES, dropping volume, noticeably in rails, nose and tail, will reduce corkyness that you experience when you get backwash or sidewash and surface chop in less than ideal conditions.

I have a few too many boards but like using them all when the vibe is right. I'll give a brief rundown of only a couple and what I've found from my experience of surfing in conditions from 1ft-10ft, and soft to quality hollow sucky waves.

JP Surf Pro 8' x 27" at 95L - surfs great in all sized waves, but comes into it's own in 5ft-10ft quality where paddle speed helps getting in compared to my 7'10" Acid. Holds in the face of steep waves if positioned right, and fits better in bigger waves so allows going more vertical in turns off the bottom towards the top of the lip. Gets hung up in this style of surfing in smaller waves for

Sunova Acid 7'10" x 26.5" at 80L - After lots of time on this board, it is my go to board from 3ft up to 8ft where is then lacks ideal paddle speed on the bigger waves but perfect at 6ft and below. I work a little harder on this board compared to the JP, but if has good stability and has noticeably less corkyness. A true high performance board that rips in sucky hollow waves and will handle and make anything that the user can throw at it with enough skill like knifing very late sucky take offs angled into the pit. By having reduced volume in nose and tail compared to the JP, the Acid at this size can be pointed towards a wave (me facing out to sea) and I can spin it much later to catch an incoming wave. Less likely to get bucked off by backwash, sidewash and chop due to lower volume sitting more in the water. Lower nose volume allows punching through chop and lips of waves when paddling out than the JP.

Sunova Acid 7'4" x 25" at 70L - Hard work for me and I can only ride it in groomed conditions to 3ft so far. It RIP's but I'm not pro enough to get the payback (yet ). I rarely ride it but will not right it off.

Sunova Insane 8'11" x 26.5" at 102L - The 8'5" would have been better but I found it cheap so why not. It excels in juice, 6-10+ft, as it's super sure footed when dropping down into meaty bombs. Does what it's meant to: turns great with predictability, good paddle speed to get in early when most are struggling in the big days, holds in the face with confidence. The volume is only a hinderance for me when it's bumpy and messy as it bucks me around a little.

Final note: All these boards allow for getting out in bigger conditions as they are still on the lowish side of volume and I don't get dragged back to the beach too much to not make it impossible. A high amount of fitness, skill, and timing is still required though.

Take the leap, and don't be afraid to make bigger increments in volume drop. The brain will train itself to cope after one hour if you are pretty physically conditioned, and the waves you'll surf well will put a smile on your dial forever

I would be very careful about dropping length below 8ft unless its a small weak wave specific board with wide nose and tail. I would try a performance shape 8'4 x 27 x 90 liters+- if you are ambitious and really good balance. I dont know 71 a high performance but still low volume longboard might be better. Length allows you to keep thin rails, narrow for hold and rail to rail, and a pulled in tail without it being so unstable its unusable.Yes low volume isnt bounced around as much but it can also be harder to recover once you get off center. Try first you might be surprised how much small changes affect balance of board. Forget everything you know about board length from shortboard prone surfing, 8 ft is really short for a performance sup for most people.

Good advice by others.
I would add:

[1] I love small SUPs, and especially short ones, but they are worth it only in small glassy waves. Any bump or chop and they become hell fast.

[2] As I age (I am "only" 65yo), length becomes also critical for ease of "pop up" (actually standing up). The sweet spot to stand on becomes critically small under 8', and the stiffer you are, the harder it is to land your feet there. I was using 6'8" SUPs at 97kg without problems, but after 63yo I realized I needed 8' to not fall when standing up. We are all age differently... but we all age :-)

[3] Volume by itself does mean nothing, always compare the board volume to the rider weight (ideally the total weight: rider+board+paddle+wetsuit...). A good ratio is the simple "guild factor" (GF): Board volume in liters / rider weight. This means:
- Your spice: 106 / 67 = 1.58 GF
- Your planned custom: 95/67 = 1.42 GF
- Your goal: 85/67 = 1.27 GF

[3] On volume, I just stumbled yesterday on two interesting videos of two people SUPing in the same kind of conditions (bumpy Hawaiian surf).

The young pro (Zane) is at a GF of 0.94



The very good, but non-pro older Wayde is at GF 1.34



Of course, on gentler and cleaner surf, you can comfortably get to lowers GF, but you do not need ultra-small GFs to rip.

Hey oldnut, if you’re looking to replicate a shortboard experience then you’re on the right track.
As you drop volume there’s a huge uptick in responsiveness and “feel”through the board.
you’ll always find a way to get the thing back to the lineup and into the wave; in my opinion the focus should be on performance.
I’m 77kg and at 103 litres my 8’1 x 28 JP feels like an aircraft carrier compared to my go to 7’6 x 27 x 92 litre in the same model.

It seems experimenting and persistence of different sup lengths and volumes is desirable. Difficulty is avoiding paying a fortune for new boards to do that. I don’t see many secondhand sups around the 100 litres or less for sale. I’ll rethink the custom based on the advice to date, to around 7’10” - 8’and 95 litres I think, gives me a gf 1.47 but I guess thinner rails, even step down rails, and see how that goes before dropping further. I certainly want less volume than my 106 litres now.