Danny,
Let me make sure we're on the same page. You've already drilled and tapped the barrel for a 5/16-24 liner and installed the liner you made but there's nothing for it to seat against, so it will thread on into the bore. Also, not enough room to go up to a 3/8 Whitelightning. Correct?
Can you make a new 5/16-24 liner with a little shoulder and countersink the hole in the barrel slightly to accommodate the shoulder? Ala Whitelightning liners.
Or make a new 5/16-24 liner but don't thread it all the way. The unfinished threads will act as a stop. Screw it in tight and face it off on both ends.
Or, I had another thought but now hesitate to mention it... aw what the $#*!. Screw your liner in to where you want it, cut and file off on the outside, then center punch each side of the liner where it meets the barrel to lock it in place. Face off any of the liner hanging into the bore. I think this would be a last resort.
-Ron