John,
The switch from disk to drum is easily accomplished with many kits available on the market. They're not cheap, however.
Power steering isn't hard to get a kit for as well.
Gas tank relocation shouldn't be hard if you can get a tank. I assume you'd want to fit it between the frame rails, so you'd be looking for a Suburban/Panel Delivery type tank.
The seat is trivial, especially if you're handy with a welder - I've seen just about anything swapped in from '70s passenger cars to modern Avalanche/Trailblazer.
Brian
