Well none of the swaps are going to be easy or cheap. With a 20R I am assuming that you have a very early truck, so you probably have an L series transmission too. I would at least replace it with a W or R series tranny, but it would probably be better to use a domestic transmission that goes with the motor. Jay K has a lot of good swap info on his website. I like the 4.3L swap because they fit well, are lighter than the V8s, and make good torque.

Another option is to use a Toyota motor, such as the 5M or 7M motor from a Supra (you would probably have fitment problems since these are long inline six cylinders) or a 3.4L V6 from a Tacoma. Or if you want to have a lot of work buy a truck that already has a motor swap, like Brian Gallus's pickup.