Agree... that is my concern as well. I don't do metal work, neither have the space in Florida, but will look for somebody that does, to complete that part. Maybe some square tubing for axles or angled iron
The center ramp is 18" wide, the lateral ones are 11".
I could reduce that axle from 36" to let's say 28 having the tires closer to the choc... there would be some trade in stability... thinking in use angle iron to support the axles all the way could work nicely.
The diameter of the wheels is needed to keep using the same ramp i always used (8').
Fun experiment at this time
Version one works... but I need to improve a few things.... the cart needs brakes so it can stay in place while putting the bike on top of it... I had to wedge a couple rubber things to keep it in place... also my contraption to hold the winch is sturdy but will be better to have it secured in the front of the bed instead and simplify that... good exercise, but moving to a version 2... learned a lot... maybe the winch should be in the cart and just an strong anchor point to pull me in the front center of the bed...
I admire your determination and all round effort to achieve your goal.
It's quite a task, getting a big heavy bike all the way up and onto the bed of a 4x4 pickup. So many things can go wrong in the process.