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.

[Palf70Step]

I have never don what you are suggesting, so no help. Hopefuylly someone else may have run across this and can feed you the info.

[b454rat]

Pull 305, install 350. I had a 96 centurion that had a 305. Had issues with the motor, thought spiders where bad. Wasn't gonna drop couple hundred for a 305. Found donor truck with a 350, pulled motor from 96, ran the numbers and found out was a 350. Installed new 350, didn't/couldn't change computer cause mine was 1/2 ton and donor was 1 ton. But motor ran fine and no major issues. The exhaust wasnt changed and before an o2 went bad there was no lights. I couldn't check the computer if it was swapped, numbers off it were gone. I'm thinking they weren't, cause light would be on looking for the other sensor....

[95 S_Trucker]

I would just use the exhaust from the 305. It would save you a lot of trouble. You could use all of the wiring and PCM that is currently in your truck.

If you think exhaust flow will be an issue, Get a new y-pipe made with larger pipes, but keep the Y and go into 1 cat.

[Tom]

No need for 4th O2 sensor. Plug 350 in and go faster.

[manimal]

Ok, a little update. A friend of mine gave me a 5.7 Vortec...#s 5/6 rod bearings spun. So I tore down the engine completely. Found one of the heads is an aftermarket head, no casting #s and screw in studs...no idea how to identify whos head it is. So it needs a crank and 2 rods. Went ahead and had the 305 heads done just to get the truck up and running again while I build the 5.7.
Now as for the ECU(computer) Dont I have to have it flashed for the additional cubes? For the exhaust I will have a custom Y-pipe made from mandrel bends and keep the 3 O2 sensors. I never really understood the 4 O2 sensors in the first place.(I know the dual cats, but whats wrong with only 1 cat?)
Thank You for all the help.