Goonee, don't waste your money on a crate engine. You can get a good used low miles 3.4 out of a '93-95 Camaro or Firebird from any number of wrecking yards. Try www.car-part.com
for starters. Check the Isuzu BBB at http://www.outdoorwire.com/4x4/isuzu/
for some good info on 3.4 sources.
You don't want to use the GP motor, it's a totally different breed of cat in the FWD and would require extensive mods to install.
The Camaro 3.4 will bolt right in to your Blazer using all the appropriate 2.8 parts, including the intake manifold, distributor, oil pan, oil pump (but put a new one in), valve covers, timing cover, etc. The '89 2.8 should be new enough to have the correct-balanced flywheel for the 3.4 (the 3.4 is an internally balanced motor whereas older 2.8's are externally balanced and their flywheels won't work with a 3.4).
Bore out the intake to fit a GM 4.3 TBI or Holley Big-Bore TBI and it will run even better. The 2.8 EFI system adjusts pretty well to the larger motor, but it does work better with the larger TBI. You should be able to pick up a used 4.3 TBI pretty cheap. Or the Holley at Jegs for around $315.
Go for it, Dude, the 3.4 is way smoother, and as was said before is much lighter than the 4.3. It's the most bang for the buck out there.....ed