I would keep the 2800 stall,perfect for these cars,best time I ran with the 4speed was a 11.2@126.9 1.89 60ft,then we put a 2spd powerglide in it,thought id go with a higher stall in it! went with a 3800,ended up going backwards ran a 11.7 @121mph 1.87 60ft,also power went from 380rwkw to 310rwkw wasnt happy at all!so we put another 2800 back in it,power went back up to 375rwkw and ran a 10.85@126mph 1.63 60ft!all this was done in similar weather(ie temp)conditions,big stalls do rob power!and top end speed.but since you didnt run it at the track 1st you might not notice it,hope this helps cheers lee