How much do you want to spend?
The easiest way to get EFI, is an aftermarket EFI system. They come with their own wireing/ manifold/parts/adapters. Most are a fairly simple bolt on install with simple wireing needed. Depending on which kind you buy. You will pay $1000-$4000
Holley, Edelbrock, Accel, & FAST EFI, all make systems that will work for you
OR
You can get a stock GM TBI (Throttle body injection) or TPI (Tuned port injection) system and wireing harness from a doner vehicle. It is usually cheaper than aftermarket systems but more work.
You will be doing a lot of wireing, adapting and need a chip burned to your engine specs. Or a tuneable ECM for your engine management which will add to the cost.
A stock TBI conversion will be around $500 or so after everything is done.
TPI is a better system but usually will cost more unless you find a good deal on the whole system.
Holley is susposed to be coming out with a new system called Avenger EFI. They will have both TBI and multi port systems. The TBI system is a direct bolt on to a 4 barrel square bore manifold and self tuneing. Susposed to easy to install and plug and play. I imagine a Avenger TBI system that supports up to 400 hp will be around $1600
