I turned a 12FV .223 into a fast twist (1:8) .22-250 to shoot 80gr bullets and absolutely love it. The blind mags are nothing more than an external mag with the base plate pulled off. If you just wanted to find an original internal setup I bet somebody on this forum has one lying around as a ton of 12FV were bought.

But the thing I liked most was going away from the internal mag...always hated loading that thing. I put mine in an Oryx chassis and have not looked back.

The other thing you will need besides the bolt face is the shorter baffle so you can get a full bolt stroke. Both parts are cheap...

Look forward to seeing your build...good luck!