Chevy had negative ground, GMC was positive. If your jumper cables are heavy enough you should be able to turn the engine over. Do not crank for more than about 15 seconds continuous to avoid overheating the cables and/or starter. If the generator is working, you can disconnect the jumper cables and the engine will continue to run, don't try this with an alternator.
Battery size, iirc is a 24. To be sure, measure the battery box and buy one that will fit in the hole. Don't forget about the height, you don't want the cover shorting out the posts.
