I never loaded the 450 a-frame, but got my best results with RL 15 and Varget with 500 gr bullets lit off with Fed 215 primers. The only 450 gr bullet I tried was the barnes tsx, and it's such a long bullet that you get the effective powder capacity of a 458 win mag.
If you can't find published data, my suggestion would be to use data for 500 gr bullets and carefully work up from there.