Hello all new member here, I'm rebuilding a 69 firebird and have been on the fence for a couple of years regarding the engine, it currently has a 400 with th400 in it but I was always thinking of putting an ls in it. So the decision has been made I'm currently rebuilding a LQ9 6.0l but dont have a tranny yet....my question is this, can I slap a 4l60e on this engine or should I put a 4l80e. This will be a street car not really raced or driven hard, i might put the pedal to the floor once in a while. Looking for advise on weather the 4l60e will hold up to the 6.0l. As far as hp goes I will keep the 319 heads 10.1 compression ratio maybe mild cam but pretty much stock might try to get 400hp out of it. Thanks

Ok thanks, stupid question here (get used to it I got lots) my LS is from an 03 silverado, my buddy has an 4l80e from a 1992 gmc 3500 would this fit or should I be looking for one from a gen III vehicle, also would a 4l80e fit in my 69 firebird?

I don’t think it will work... maybe wrong. A built 4l60e will last, but I think it depends on your budget. If you’re talking a stock 60 vs a stock 80, I’d go with the 80 for insurance. Especially since it’s a street car and you’re not overly concerned with extra drivetrain weight.