2003 Suburban 5.3 upgrade or LS6 Swap

Hello to everyone here. I have been a lurker here for sometime. Lots of great info.
I decided to keep my 03 Sub and just do some minor work on it versus getting a new truck. I imagine there are several builds on here that started just like that. After having the motor pulled yesterday and discovering the cam bearings are locked onto the cam and have eaten away the block. I realized I need a new block. The reason the motor was pulled in the first place was no oil pressure at idle. The shop that is doing the work has a very nice, shiny aluminum LS6 block on the floor right now.
I already have a new cam, springs, retainers, shorty headers for the 5.3. Nothing major, this is a daily driver that sees lots of highway travel between Texas and Colorado. Parts are Comp Cam 54-452-11 (212/216@.050 520/524 115 lsa) 918 springs and steel retainers.
The real question is can I use the 5.3 heads with the LS6 or will my compression go through the roof with the smaller chambers? Stock 5.3 chambers being 61cc. I don't really want to spend the money on new heads. I am guessing I can drop some of the compression with dished pistons. There seems to be lots of talk of doing just this on the corvette forums for the added compression, but I want to run cheap gas and be able to run the motor for 20 hours straight up and down mountain roads.
I hope this is enough info.

Off topic from what you're asking, but back to the issue with the oil pressure, I have the same problem right now with my 04 5.3. No oil pressure at idle but around 20 while driving. Did you notice anything else, noises, etc, before you pulled the motor? I'm trying to see if maybe I just need a new oil pump. Thanks.

I had no indication of this problem until I stopped for gas during a trip from TX to CO. I was somewhere in NM. I pulled up to the pump, the pressure dropped, and the alarm started. I checked everything. No leaks, knocks, etc. I finished my trip to CO. I had a new oil pump put in with new o-rings and pick up tube. I also put 2 quarts of Lucas oil stabilizer in, but that did not help. Once the truck gets to operating temp, pressure drops to about 4psi at idle, which sets off alarm. Running I had about 20psi. Punch it, and it would go to about 35psi. If you could keep the idle at around 750, pressure would stay high enough to keep alarm off.
So to answer your question, there were no indications of a problem before or during. I drove it 1100 miles home from CO like that and had no other issues. If your problem is a spun cam bearing, like mine, I would get it fixed. It only gets worse over time. I would hate to imagine what the next step would be in that line of failure.

Wow, that is exactly the issue my truck is having. I haven't changed the pump yet but my gut was telling me that wasn't the problem. Well, looks like I might as well just bite the bullet and get a new engine.