If the heads are 243's and are stock on a 6.0L engine, you are looking at an LS2. The 243's also came on LS6 engines, but are 5.7L engines. 

If you are actually wondering which engine came *out* of the Avalanche and it has 243 heads, you are looking at a 5.3L with some other than stock heads. They didn't install the 243 heads on the truck engines. 

You should be able to source *any* truck wiring harness and it will mate up. If you are going to put a car accessory bracket up front, I suggest you get a wiring harness for an LS car (97-02 Camaro/Firebird). I don't think I'd try a GTO wiring harness, but it could work.

As for the 94 T-56, it **will not work behind the LS motor**. There are a bunch of differences besides just the bell housing. The clutch release (throw out bearing) is completely different (push instead of pull). I believe the input shaft is different as well. You need to find a T-56 out of an LS car (Camaro or GTO) for it to mate correctly.