#4 threads are smaller than the P14 - NP with the conversion.-- The threaded section is also shorter as well - might need a collar to be fitted nicely to improve appearance and cover the extra thread/s. Been a long time since I did the re-barrel on the P14- .303 chamber in a .308 groove bl. - Larger than standard Mauser & square threads as well - IIRC.

As to feeding in the P-14 - the rim of a .45/90 is .068" larger than a .303 rim. To get the height right at the rear, at least that much must be removed from the action rails to raise the rim. I'd think that much would also have to be removed from the parallel - rim port in the rails too.

Ctg. nose diving sounds as if the rim is dragging heavily on the back of the mag follower.

I'd turn a rim slightly to see if that helped - maybe use a .45/70 case with a long bullet seated in it and turn the rim down to about .560" from .608" (instead of using/wrecking a .45/90 case).

Try turning the belt off a magnum case and see if that will feed from the P14 mag. The .532" rim is just a bit (.008")smaller than a .303 rim. If the mag case feeds it will show the rim alone is causing the grief.