Need to now what size this block is

Hey Sherman, look on the rear of motor just above tranny passengers side. that number is what you want.

keep the heads and put them on the 350.. it will give you great compression...

ECM is ELECTRONIC CONTROL UNIT

It can be done. It's the same block as a 350 so you should be able to use the same parts as a 350 swap. It's a tight fit for sure and requires some specialized parts. Look around on the internet for 350 swaps or check some of the off road mags. You shouldn't have any trouble finding complete swap kits or step by step instructions as well as what type of special pieces you will need. Hope this helps.

LOOK AT YOUR VIN NUMBER THROUGH WINDSHIELD 8TH NUMBER FROM LEFT TO RIGHT ENGINE CODE OR LOOK ON YOUR VECHICLE REGISTRATION. VIN H 305 ENGINE AND VIN K 350 SOME TIMES 305 OR 350 BE ON THE INTAKE MANIFOLD OR FOUND ENGINE CASE PAD FRONT RIGHT HAND SIDE OF THE CYLINDER BLOCK.

if this is a fuel injected engine youll most likely have to replace the idle speed control sensor this should solve that problem.youll have to get one for the intake that you put in the car(one for the year and engine size of the replacement parts)

Not a stock 1991 GMC Sonoma transmission. They're made to fit a V6, not a V8.
(With a 4.3Liter V6 engine)

almost every rear drive drive full size used the 305 V8, so u can get an engine from a Buick La sabre, Pontiac bonneville and firebird, also Oldsmobile 88, and don't forget Camaro. any 305 chevy V8 will bolt in, also the 350 V8 will bolt in, this is a much better choice..

are you going for extra horse or just to have a bigger block. you can do a lot with a small block to give power. change that cam and intake and add a nice 4barrel and some headers and a thrush exhaust and if money is not a problem some new heads . power is in the heds. you can go on line and find vedeos that will show you how to port and polish your heads and you get bigger valves both intake and exhaust been there don this and you can make a bad *** motor

good idea, use 350 block,305 heads &intake.the reason is, if the 91 305 heads are 4 bolts in center of valve cover the are what you call vortec heads,they have better flow.the tbi will respond better &still have bettergas milage than old style heads.the 350 block will add more overall power.you shouldnt have any problems doing so,you will notice a diff.have fun, good luck.