Let me ad a small story to show you the importance of proper convertor sizing. Here's my Sons combo

70 Duster (3100 pounds)
4.10 gears
28-10 slicks with caltracks
.040 over 360 engine
11.1 flat-top pistons
stock crank
ported Edelbrock heads
ported Victor intake
750 alcohol carb
904 with 5600 stall convertor

His best run with this combo was in the 10.50's

A transmission line broke and fried the transmission so we installed a fully prepped John Cope Racing transmission and one of two convertors I bought from a Moparts member. Next time out the car slowed to a 11.20 with a just freshened convertor that stalled at 4500 rpm. I called and talked to John and he said the difference between a 904 and a 727 is minimal with the PROPER convertor. We pulled the transmission again and installed the other convertor. Next time out it stalled at 5400 and ran 10.50's again and has since run in the 10.40's. These engines LOVE high stall convertors.