Evan,
to base your rifle on a P14 you've picked a winner as in reliability & simplicity of action, field stripping etc, a LOT of work still to be done though!
Have you milled the action to hold both sides of the Jeffery round without it popping up as it sits on the follower; & hence only reloading from the bottom?
The new mag well is HUGE, but to get 4 in there is a real feat in itself!
Your follower I guess could be then be conventionally shaped to allow for the staggered stack - just much larger?
As for an 18" barrel & 8 1/2 lbs, you are one brave & tough sod, that'd kick the snot out of me!
Imaging the muzzle flash just on dusk - that would be epic!