You'd better research the emissions testing in your jurisdiction before you go any further. Is is a sniff test or does it include a visual requiring all oem equipment to be in place? You may pass the sniff test, but the second one you are out of luck unless you can fool them, or buy an approved system like Howell's.
For GM tbi I see three options:
1)Buy a complete system, Howell, Turbo City, Affordable Fuel Injection, etc. Install and turnkey.
But you want junkyard right?
2) a) Install a complete 4.3L TBI system by extracting every part and whole harness from a donor vehicle. I know a guy that did it, and he says it works great. See this source for some info:
Do it yourself page
For best results, you need to modify the chip. These systems all estimate air flow by a method known as speed density, which relies on accurate tables of volumetric efficiency of an engine at various rpms and MAPs (manifold absolute pressures). A 258 is very different from a 4.3 in torque curve and would be probably lean at peak torque rpm and rich at a higher rpm if you used an unmodified 4.3 chip.
b)Buy the chip, harness and adapter from Howell and use a junkyard ECU, tbi and whatever else you care to scrounge.
c) Burn your own chip. See the DIY EFI group.
3) Go full experimental mode and use a 4.3, 305 or 350 TBI (all the same size venturi, only the injectors vary in fuel delivery capacity, measured in lbs/hr) and a universal controller like Megasquirt
Megasquirt frequently asked questions page
The only way you will pass testing with the megasquirt is if they only check for proper emissions, and even that is bound to be a challenge, although it has been done.
I live in an area of Canada where there are no emissions testing and I am using Megasquirt on my 258. I've very happy with the results and the fun of the ongoing tuning. Do a search here for Megasquirt and there should be a long thread where I have some info and pictures. I think it dates back to January. Mike, who frequents this board is in the midst of getting his squirted 258 running.
I'd also like to see someone try a 4.0L intake manifold and injectors and fire them with the megasquirt. It should work as well, although the squirts will not be sequential. This will make little difference except at idle. Lots of multiport engines run very well on batch fired injectors and idle just fine. Emissions might be worse though. I am told no difference at speed. If I had another engine, I'd try it myself. On the other hand, the tbi system is working well and delivers superior horsepower to the carb, so I have to wonder, why bother, except to satisfy my curiosity.
Tom, who frequents this board, is putting together a boneyard based GM tbi system, except he has a spare Howell chip to run in it.