5.9 is a bolt in and will run fine with the stock 5.2 confuser.

That engine is supposed to be 9 to 1 already, probably 8.5 in reality.

I built one where I square decked and zerodecked the stock pistons and ran a .039 fel pro head gasket for a while and with everything else bone stock it picked up significant MPG and TQ, got the MPG of a V6 with the power of a 5.9. I later switched to a .028 head gasket and my dad is driving it but he don't keep track of MPG good enough for me to know if that helped any more. But it runs awesome with no pinging on 87 octane and it is over 10 to 1. I would not hesitate to build one like it again. A little porting and a better flowing exhaust would probably help even more. If you plan to follow that build and want to upgrade the cam you will probably need notches in the pistons as P to V is very close with the stock cam. Do some porting in the throttle body and a CAI as well. Any serious mods are gonna need to do something with the beerbarrel intake, it heatsoaks real bad.