I have to agree on the TBI.
Of course, you may still ahve to deal with the '86+ manifold issue, but the setups are out there fairly cheap, and if you find an exact donor truck, you use everything off the donor.
The expense in fuel injection swaps are the things that aren't included, or need changed due to engine mods.
IE, have to get a new fuel pump, plumb new fuel lines, new air cleaner setup, different distributor, wiring, etc. Engine mods might be different cam, or different heads that won't allow you to use the stock TBI intake, and so on.
Absolute cheapest way WILL be to find a donor truck that is 100% complete in regards to the fuel, electrical, and mechanical systems.