16.5's are still available from Firestone to name one, some for trucks, more for trailers and lots for skid steers. My '74 had 16.5's with one piece rims.
The fenders can stay on. You'll need a floor jack, short chunk of 2x4 and a helper to steady the unhinged door while installing the replacement hinges.
My vote also is for new hinges, both upper & lower. I never could understand the thinking on the thru the dash hinge bolt. My '55.1 has that setup for the hood hinges.
If you can salvage the cab mounts off of the p/u frame and use them on the sub's frame it should work. Use the fuel tank from the sub, make a flat bed and your good to go.
That don't look right. I've never seen an adapter plate that would make those fit. Are they correct for the engine/heads? I'd find a SBC iron head to verify the spacing.
My vote is to replace the clutch. Stay away from the three finger style pressure plate and the hockey puck type of disc, also replace the pilot bushing.
Should I have recommended to dump the clutch?
My '74 came with the glue down type dash cover and I've used them on a couple of mid 80's blazers. They fit ok with a little of massaging and a heat gun. The last one came from LMC and they are cheaper than a replacement dash pad. None of my vehicles are 100 point winners.