I replace my fuel lines with fuel grade rubber hose.. works absolutely fantastic, and easy to install in less than 30 minutes.. just make sure to secure the line away from any moving parts and heat sources.
As far as checking your tank for leaks, isn't the tank in the '51 Jimmy behind the seat like it is in the Chevrolet trucks? If so, you'll know you got a leak if it stinks like gas in the cab.
If it's been sitting for awhile, I recommend pulling the tank out and flushing it out with a handful of rocks in there to knock loose any rust.. then you can seal the tank with an off the shelf gas tank sealer kit from your local auto parts store.
Any leaks you might have will be pretty obvious during the tank flushing.
I don't recommend cutting drive shafts and that kind of stuff yourself, tho.. unless you really know what your doing.. just because those things have to also be balanced after they're cut.. and they're not very forgiving if you screw up and cut a little too short!
On the frame.. lots of guys like to take their 1/2 ton trucks and sit 'em on later model or custom frames.. so it's not real hard to land an original 1/2 ton frame in good shape pretty cheap.. and that might be a easier way to go about things, if you feel like doing the legwork trying to find a 1/2 ton frame, anyway.
Heck, I have a couple 1/2 ton Chevy truck frames i'd just GIVE you if you was to come out and help me yank the body parts off of.. too bad you live all the way down in Texas.
