Your workmanship is looking really good there Ron, however, I do think you need to reconsider a couple of the design factors.
Firstly, the rear tyres look like they are perfect for the application but I have serious concerns about the rimless rear wheels.
I believe you will have a catastrophic failure of them under side load, to which they will subjected to in alternately opposite directions during the practise of land sailing.
All is not lost though. The tyres look to be ones that would fit a 6" rim. A pert near perfect plastic peripheral piece to suit 6" tyres is available at Enmach Industries, located here in Bundaberg. Click on the link and then click on Poly lawn products and you will see them.
enmachind.com.au/Click around the site and you will see lots of other interesting products like plastic fuel and water tanks to suit 4WDs, camper trailers, boats and whatever else your imagination can cook up. Their prices are quite reasonable too.
The last time I bought a plastic wheel from them it came with 4" tyre and tube (not optional) for $60. The hub, from memory, accepts 6202 precision bearings.
The rims are what blokart originally fitted to their yachts and are more flexible and less brittle than the blokart branded wheels they now use.
Enmach I believe buys the rims from a manufactuter located at Whangerai, NZ.
Secondly. I think your sheeting angle is way off.
I suggest laying your seat back back another 15-20 degrees. If you do not want to do that, move the boom blocks forward to directly above the rear frame block.
Cheers Cisco.