Yes you can. The engine blocks are the same. Just use your 99 heads and components (intake, distributor, ECT.) They will bolt up to the 93 block and the transmission will bolt up too.
I'm not trying to offend anyone, so please don't take this the wrong way. GM has always been grate about the parts fitting from one to another. As I am sure most know, you can get a motorcycle (Boss Hoss)with a Chevy V-8 on it, proving the point that you can put nearly any engine in nearly anything you want to. The point of my reply about the 5.3 is that it would be much easer to replace a 5.7 L with a 5.7 L If you do a 5.3 then you need the computer and wiring to manage and support the 5.3 The 1993 5.7 engine block is the same block as the 1996 5.7 L is. If he uses the 1993 block and uses his 1996 top engine components,(EG heads, and all the upper section of the engine) this should be a easy swap. IMO the 5.7 L (350) is the best engine ever made, and that's why GM made it for 35 years. I would never take out my 5.7 L to put an LS engine in its place, but I know people do. When the day comes for me to do an engine swap, I will do a 383 Vortec stroker, but that's just me. Again like I said, please do take this the wrong way.You can put the 5.3 in you just need mount adapters. If you can harvest the pcm and harnesses out of the other truck it makes it much easier. I don't know if these are the righ adapters for you but here is a set http://m.ebay.com/itm/271582394207