We have an 85 Beast, but we are running a 2008 496 HO MPI 425 hp. The tranny is still the 1-1 Borg Warner Velvet drive. The Prop we are running is an OJ 14 X 16 SST LH with one person in boat 1/2 fuel the Max RPM is right at 5k at 52 mph. We ran just the other day with full fuel and 3 people and max rpm was 4,850 give or take and just at 50 mph. If you truely have the 380 hp version 454 a 14 X 14 should be perfect for you, The original Mark iv 454 had a max rpm of 4,500 rpm. However most of the 454's Supra installed were the 330 hp. an easy was to check is the 330 hp had a q jet carb and the 380 had the Holley carb. Good luck and post some pics. Al
Supra Beast
Custom 496 MPI HO 425 hp
Custom Teak floor