i put a a500 into my 64----in the body of the transmission there was no interference at all--but the overdrive unit was bigger than the driveshaft tunnel--i trimmed off a couple tabs with a cut off wheel then made 3 dents with a hammer in between the seats and it was done----not noticable with the carpet----the 518 will fit in there just like the 727--but it has an identical overdrive unit so the same trimming will be required---