Well i have a F.A.S.T. drag ported 4mill 392 on alky and i have just +12 on mine i think a 10 would have been about perfect but i'll probably go with more motor one day so i got a 12. If mine needed a +10 of rht efrontend not to lift i'm sure yours will need a +12
the main problem with a long swinger on a stock frame is that the longer you make the swinger..the less weight over the back tires which= less traction...you can over tire it to compensate for the lack of proper wieght distrubution,but that is going to slow you down on the big end.
If I were you I would run anything between a +8 to a +11,with a wheelie bar.