Talk you out of it? Yeah, right.
Maybe you could take a look at a Bren mag. It's squared off at the front, unlike the PSL mag. It's made for a large rimmed case. They're affordable. They're BIG. They're thick enough to weld without blowing big holes in them.
The downside is that they have a big rib on the back for the bolt hold open lug, which might need to be ground/cut off and a new back panel attached. The mag is really just too big for anything less than a PSL bolt and carrier, but you seem to be planning on going that route so it shouldn't be an issue.
For barrels, you may want to look at Thompson Center. I believe they have 16.5" and 21" versions.