I have not heard of that happening. It would need to be an extreme change for it to slow down. For a car like yours going from 256 to 355 (forgot your gearing) or so I would think it would be faster. What rear axle ratio was it before & what is the gear ratio now?
Some engines do like to run sort of tall gearing from the factory, like Ponchos 455's , Oldsmobile, 455's etc. I think if you were to put 456 gears in those cars, they will be slower in the 1/4,,,, talking stock here.
I have seen my friends car go slower from using a Weiand "X" celerator manifold then switching to a Victor Jr SBC. .5 second difference in the 1/4.
A loose/slushy torque converter usually will loose MPH, but could ET better. The less slippage the better MPH
