The converter is not right and the headers are too big. Call and talk to Marco Abruzzi on the converter situation. He will straighten that all out. His stuff is 1st class. A good 505 at that weight out to run 5.0s or atleast 5.1x's

What size rear tire ? would make some sense if its a 34.5 on a 16x16 rim but if not 4:30 gear, 6000 stall converter and 2 1/8 headers would be the best fit, 800hp should go 5.1's no prob in that light of car also is it a suspended car ?

830 HP SBC in my 1875 lb dragster ran 1.12 60ft, 4.88 1/8, 7.88 1/4. My guess is converter as well, as long as you have no fuel or ignition issues in the new chassis.