Anyone ever convert a Mauser type triggerguard mag box to a Sheetmetal Rem 700 type.

I know PM is rolling in his grave.

What kind of steel.
Did you harden it after (maybe just the top lips)

Of course I have; use any steel; you aren't using the lips for feeding and even if you do, boxes are not made from hard steel. You aren't going to wear the lips out.
Aluminum will work too.

BRNO's came that way

what prompted the question was a mauser with really screwed up rails.

I have a Westley Richards marked commercial Mauser in caliber .303 British with a sheet metal box and a follower like the one in a P14 Enfield. It feeds fine.

I have done this with comml mausers, oversize (length and width) for 375 length ctgs. You saw off the old box with a hack saw and file it flat. Leave a rim about 1/8", this supports the new box. This oversize box fits a Hogue overmolded stock with minimal modification. You still have to work the feed rails to match the larger box. I may have a couple boxes, PM if you want one.