What to do...

A dual plane would be a step backwards with a 4500 stall and that cam. If this were my car I would put more converter in it, a good set of rear adjustable shocks, and start thinking about a trans brake. Get it to leave on the converter and hook and it will ET better. Where is peak torque? Have you had it on a dyno?

i doubt it will Et better using a brake.
I have had more motor than this and car 60 footed and ET’ed identically off foot or brake. If the convertor is right., that will most always be the case.
9 sec, 3350 pound car going 1.34/1.35 either way on leafs