The key point with mast steps is to try and NOT to create a single 'pressure point'

Figure A would be better than figure B.

The timber insert in A would be good to have a very slight taper [only about 1-2mm] in the last 50+mm of the timber just to distribute any stresses, Masts are like egg shells.... strong but brittle.

The steel mast tube welded to the yacht also needs a very gentle taper at the top, you have indicated a rolled over edge but a bigger taper would be better.

Glueing in the timber insert should be a 'soft' glue, silicone work well as it only needs to hold the timber insert in the mast when the mast is removed from the yacht.

Have a read through this.. There is a nice pic of how to do it...

www.seabreeze.com.au/forums/Land-Yacht-Sailing/Construction/Build-a-Landyacht-Lake-Lefroy-Mini-Yacht/

People have used plastic tube (plumbing pipe) to reduce rubbing others haven't it doesn't seem to be a problem either way.

When you do put the mast step onto the frame make sure that is a strap going from one side of the mast tube, under the frame, and then to the other side of the mast tube to added additional strength and reduce the stress on the welds (25mm x 2mm x half way up the mast tube)

Something also to think about is a couple of small drain holes at the base of the mast tube to let water out if it rains while in storage, rust in the mast step can be a problem.