904 or 727 in 66 barracuda

The smaller 904 case also give more room for exhaust and maintenance. A500 is 904 based but requires major modifications to transmission mount (torsion bar) cross member and floor pan to use as well as shortening of driveshaft. Biggest durability complaint I'm aware of was with the early push button shifted versions which used a smaller diameter front pump drive on the converter. Personal experience with this failure, pump drive would shear off converter, when launching my 3000# car at dragstrip at a 4500 stall speed on slicks. In normal use behind a mild built street motor I've never seen this failure even in the early 904 versions. So I don't think they are an especially "weak" transmission. My 2 cents.