I have a bunch of 45-90 cases that I would like to trim to 45-70 length, 2.1" What is the best way to go about it?

There probably is a good market for 45-90 brass. Sell what you have and buy the 45-70s or find a swap.Or not and get a trim die , hacksaw and a rough flat bastard. roger

You could probably trade the 1 for 3 for 45-70 cases and come out way ahead.

That said the easiest way to trim is to use a Wilson case holder for a 45-70 (or .284 Win). Tap the case into the case holder. Hold the case holder in a 5C collet with the case head against an internal collet stop and part them off.
Someoen with a lathe set up like this could cut off a case a minute or less.