The following is posted at the P-38 Forum, but I'll include it here for continuity...
I pressed the liner in place, cut off the excess and faced each end, chambered it for 7.65 Luger, and chamfered the chamber edge similar to the 9mm breechface...
I still need to cut the extractor notch, but this operation looks do-able...
(9mm barrel shown for comparison)
I'm waiting for the P1 barrel to come in; I imagine it will be slightly different in construction since it is already lined and pinned...