In a case that size, I'd think 50fps is about right with the slow rifle powder with light to moderate loads.

A powder used in the magnum cases noted, is probably very slow and might not give you full velocity. It is probably similar to 4350, 4831 or 7828.

Most slow rifle powders can take compression in stride, work better when and a full case to a compressed load might be necessary to obtain factory ballistics.

I do not load for a .470, but have 'some' experience loading ctg.s that have no data - similar circumstance to yours. You are lucky in having factory ammo for cross-referencing.

Measure a factory case at the web(about 3/8" above the rim) to see what the expansion is. Use a mic, not callipers if possible) You can load to that expansion in once fired brass. Go slow, perhaps 2 to 3 gr. increase at a time.

The Rem 9 1/2 mag primers should be just fine, although a longer burn as given by the Fed 215 is generally prerred. Also, since you have a chronograph and knowledge of loading, you will probably have no problems with obtaining good results. Shot to shot consistancy will show whether or not the primers are up to the task of igniting the charges. the shot to shot consitancy should improve with the more powder you use, as well, compression, if loading will permit, will help the powder burn properly. Beware that too much compression can make the bullets move forward (spring-back) from their seat in the case - something to watch for.