Rivet advise please

I have to replace the trailing edge end of a stamped aluminum rib from
a '47 Tcraft. I've cut away the corroded portion, bent metal to
replace that part and would like to use Cherry "N" rivets to hold it
in place.

I have some questions regarding what spec to look for when ordering.
A. Rivet Material: 5052 Aluminum or 5056 - does it make any
difference for this old a rib?
B. Mandrel Material: Aluminum or steel - am I correct that
dissimilar metals could cause a problem and I should stick with
aluminum? In that case why make a steel mandrel?

Thanks in Advance - Mike