I would built it on the granite mountain arms action to have the 0.750 bolt body diameter. With a standard bolt body diameter, there will not be much meat left on the bolt to hold the Gibbs rim. The other reasonable alternative is a P14 enfield.

I would suggest that you contact Ryan Breeding (Palmdale, CA), Joe Smithson (Provo, Utah) or Dennis Olson (Plains, MT) as any of them can build you a .505 that works and looks great.