How tall is the driver? How much does he weigh? What engine is he running?
Assuming he’s not heavier that 220 lbs and not taller than 6 feet
A good starting point would be 8.5″ to 9.5″ at rear of seat lip to axle
3″ to 4″ from bottom middle of seat lip (by your nuts) To steering uprights
put a couple of 2×4’s at the bottom of frame rails under seat.
If he fits just right, and reaches the pedals pretty comfortably, then I would say start drilling, fasten it up and test drive. If he likes it and it handles great, leave it alone. Lol. Fast Freddy.