Being a lock up does not make it a bad tranny. The only thing different is the converter has a clutch to lock up at cruising speed. If you want to run a lock up behind a high-po mill just drill the releif passages in the VB out so it locks up faster (buy the Trans-go TF 12 or 3 kit of your choice for details) drill the holes to max diameter or even one size bigger to really firm it up so it don't slip and burn. Also install the stiff spring to delay engagement untill a higher speed. I have built several for my various cars and a couple were race cars, all were strong street cars at the least and the only one that I ever had fail was one where my cooler line blew on the way to the track and I kept driving it, it wouldn't hardly move when I got there, I raced that night after fixing it at the track. And the next day the forward clutches gave out, I rebuilt it and used the same converter and never had another problem racing that trans till I sold the car. If you build the trans right the 904 is not a problem and LU is not a problem.

PS ask Andy Mayes about the LU 904 in his roofing truck I built with a TF3 kit. He still cusses at me for building a manual shift trans for a work truck, I just did it to see if I liked it and then he bought the truck from me and is still driving it years later, says hes going to put a 727 in if he can ever kill it