I have a Vanguard 30-06 that shoots really well but the bottom medal is really junky stuff.
Would a mauser bottom medal adapt without
an extreme amount of trouble?
The box is shorter on the mauser but the howa has a mag length box on it to start with.
Weld up one bolt hole and relocate it just a bit and it should just about go.
Has any of you guys done this or is it just insomniac ramblings?

Call Legacy Sports, they will sell you a new bottom metal for not very much $$$.

This is one of those things that seems simple, then turns our to be a tar pit --

you have to worry about
screw location
screw angles
lengths and geometry of mag wall thickness, outside of front of mag to screw,inside of mag to feedramp, inside of mag to back of action, widths (in and out) back of mag to trigger clearances, trigger travel, trigger depth, then rear screw

and then the angle that the mauser is built to vs howa

and finally, the finish, as you may have quite a few welds to blue over, which may mean you have to heat treat to a like, uniform, hardness

but, it can be done -- too bad it isnt the question of a mauser bottom metal on a springfiend .. thats EASY

All the above is very true; but I am not known for being the shiniest utensil in the drawer.
Bolted the Vanguard metal up to the mauser yesterday. Magazine width seems to be the main issue.
When I run across a cheap bottom medal I am going to try it out just for kicks.