The 61 Chevs had coil springs in the back, torsion bars in the front. Pretty sure the GMC had leaf springs in the back but not positive. Yes it is possible to swap to front power discs, parts are available for a 5 lug or a 6 lug setup...there are aftermarket parts companies like LMC that sell conversion kits...I don't know how good of quality the parts are though. I'm sure there's other companies that sell the parts. As for converting it over with parts from another year of truck, I'm stumped, but if you find out let me know! Also, is your truck a standard or auto? If its a manual like mine, it likely has the hydraulic clutch, which works off the same master cylinder as the brakes. There is aftermarket power brake boosters for this that accomodate the hydraulic clutch, but man are they expensive. Maybe decide on if you're gonna keep the original powertrain or not before converting to power brakes.
As far as the tunnel cab floor section...not entirely sure what you're talking about...I'm fairly new to the hobby.
