My point on the mounting is; metal to metal mounting is going to wear into the rad tanks where they contact the rad support, and flex the rad core @ the upper anchor points.Either of which will lead to leaks, the rad coming loose, and rust stains at the contact point with the rad support as that nice new paint wears off.
Rads should be rubber mounted @ tanks or suspended, with a large contact area on the horizontal core struts, or a combination of both.
If your interested, I'll get you a link to some of the inexpensive trick mounts used on circle track cars with these type rads.
Get inside that pump.
As the problem is likely water flow related as rpm's increase, (your pump is "burning tire", making a whirlpool, instead of propelling water)
Your rad and fan
seem to have plenty of capacity. As your temp pulls right back down once the rpms come down.
Hope you get it all sorted out, hey it could be much worse, your temp gauge could look like a compass telling you your headed due East!
