Engine / Transmission Swap

I have a 1988 GMC Sierra C3500 with 5.7L TBI engine. Great Truck - we call it the work horse. The old horse now has 210K miles on it and its about time for a new engine. I'am looking for someone who has experience or information on swapping the engine and transmission. I'am thinking about a newer 5.3 or 6.0 Vortec engine. I know I will have to change the ECM and harness ,etc but hoping someone can tell me which engine / transmission combination would be the easiest fit.

Thanks
Phil