I guess it depends on how much fab work you can do yourself. The 4t80 fits and has been done on a few LS4 cars, but it's not a simple plug and play.

If you were able to do the fab work, wiring, and tuning yourself then you'd probably come out ahead given how cheap junkyard 4t80s are. If you had to start farming that stuff out, it would get more expensive pretty quick.

Only reason I bring up the 4t80 is that I think it wouldve been a better choice had GM put it in. I don't necessarily think it's worth the hassle to put it in in your case, especially if you really do have 155K miles on an untouched 4t65. Even if the trans died tomorrow and the rebuild only lasted 100k, the whole car is going to be wore out at that point anyways.

 

I wouldn't know how to do fab work to save my own life.

The rebuild lasting 100K wouldn't bother me. Not being able to pay for it is the problem. If the 80 is cheaper, it's cheaper.

 

Well that's really my point, buying a 4t80 from a junkyard would definitely be cheaper than doing a proper rebuild on your current trans. The problem is paying someone to do the custom work to make it fit and tune it is going to increase the cost way beyond just doing a rebuild on your current trans. You also add the unknown of a used trans.

Sounds like you're cheapest option will be to either save to rebuild your current trans or just scrap the car when it blows. Certainly some 4t65 make it to 200k and slightly beyond, but you're definitely coming up on the end.