I too am looking at doing a 450BM. I have a blank on hand ready to send off and have made into my barrel, was going to do a 45ACP as a single shot plinker, but the 450BM looks like a lot more fun. I was thinking I'd use a centerfeed, I have just had so much better luck with feeding from a centerfeed.

I am waiting to locate dies and brass before sending my blank off to be done. Should be a real nice short/mid range thumper.

I would do as the other gents are saying and test your SF action. I had a LA setup for the 30-06 that I put a 308 barrel on and it suprised me and fed 308 beautifully, so your SF-SA might do the 450BM well. Don't know til you try.