I gotta agree. The most expensive part of this 2WD to 4WD conversion is the drivetrain/axles, etc. BUT, like you said, you have all of this already. All you really need to add is the front spring rear shackle holes in the frame, a front spring mount to weld to the front of your 2WD frame, and mounts for your crossmember. I'd say that I would go the same route you are thinking. And yes, the first gen trucks are my favorite too!

85 4Runner - With NEW Marlin 4.7 Gears!! It's a whole new beast!