I didn't do it, but one of the guys at the ITOG site who did the 3.4 swap in his Trooper put a 4.3 TBI on. From what I recall he got a used one from www.car-part.com
for around $50.00. Another guy posted that he could get a remanufactured unit for around $75.00. Either way a lot cheaper than the Holley. I have the Holley, got it from Jegs for around $300 but that was last year; they are around $315.00 now. After some research I realized that the 4.3 TBI had the same exact bore size as the Holley unit. I went down to the local auto parts store and asked for a base gasket from a '90 S-10 with a 4.3 TBI. Lo and Behold, exactly the same bolt pattern and bore size!
When John G. over at ITOG put the 4.3 on, he said there were minor differences with the throttle cable bracket, and he had to use the 4.3 bracket to make it work right. Also, the year 4.3 TBI he got (a '92 I think), had a different style Throttle Position Sender unit, and the connector plug was different. All he did was go to the wrecking yard and pull the plug off of a wrecked Blazer's wiring harness and adapt that to his 2.8 harness. Same number of wires, just a different style of plug. Also, he said that they did change the TPS on the later models, so maybe if you found a 4.3 TBI the same year as your rig it might have the same style of equipment. The injectors and all the other lines/connections hooked up just like the 2.8 TBI.
With the stock 4.3 base gasket as a template, you can mark the holes and have a machine shop bore out the manifold for you. There is plenty of meat in the intake, and no water passages are near enough to bother with when boring the holes. Either the Holley or the 4.3 TBI will bolt right down on this modified intake with no problems. I imagine if you didn't bore the holes out, the throttle plates of the larger-sized TBI would hit the intake when you gave it the gas. This is why Holley includes a funky adapter gasket about 1/4" thick, which transisitons from the Holley to the smaller holes in the 2.8 intake.
Of course this limits the airflow somewhat but it does make installation easier since you're not pulling off the intake to have it machined.
So if you spring for a used 4.3 you'll have to bore the intake, since I don't think you can get the step-down gasket separately from Holley.
The Holley has higher-flow injectors than the stock 2.8, and the computer adjusts for the correct air/fuel mix once it's in closed loop. The Holley does have the advantage of having an adjustable fuel pressure regulator, if you need to lean it out a bit or richen it up. Most stock 2.8's will run a bit rich with the larger TBI. With various mods that make the engine breathe better, this is not so pronounced. My tailpipe on my Trooper is usually sooty, and you'd think it's running way too rich, but it was getting almost 20 MPG on the hiway last summer. Not bad for almost 4000 lbs and 4:56 gears! I expect that the injector flow rate in the 4.3 TBI is real close to the Holley's, and that's why it will work with the 2.8 EFI setup.
And if you ever swap to a 3.4, the larger TBI makes it run even better. The extra airflow demand leans that puppy right out, too!
Anyway, hope all this helps you decide which way to go. Post with any other ques you may have. good luck.....ed