1998 5.0 vortec to 5.7 vortec swap...questions?

[manimal]

I have a 98 K1500 that has a tired 5.0/4l60e. I have been chasing random misfires for about 6mo now. I have replaced the fuel pump, injector spider(Std Motor Parts), cap, rotor, wires, plugs, distributor gear, had the crank relearn done and still have the misfires..Mainly #6, but 3/5 had a few. They would clean up and go away off idle. The mechanic told me he thinks its a compression issue, mainly valves. So, I pulled the heads on Monday. The exhaust valves in those cylinders were pretty crusty and the lifter valley was full of sludge(truck has 194k).

So I have decided instead of rebuilding the heads and all the costs involved, that I would like a put a 5.7 in its place. I have done some research and have found that the exhaust is different between the two...mainly the 5.7 has "4" O2 sensors where as the 5.0 only has "3". So here are my questions...
1)Could I wire in the 4th O2 sensor into my existing harness, or do I need to swap harnesses?
2)Does the harness matter if its from a 2wd or 4wd? and is the 4th O2 sensor the ONLY difference in the two harnesses?

I have found a guy local that is parting out a 98 Tahoe with the EMC, Harness and the exhaust all for $150...BUT it's 2wd.

Sorry for the long post and Thank You for any help.