I bought a couple Indian drill rifles. The only thing I see that has been done to demill them is a hole drilled through the receiver/chamber and a pin inserted and tacked on one side. The bolts are complete and have not been modified.
While I wouldn’t trust the receivers enough to just rebarrel them in .303, I was thinking of .45acp.  I would think that with the relatively low chamber pressure of the .45acp it would probably be a safe conversion.
Thoughts on this?

zero issue doing so.  The original DeLisle had the 45ACP chamber set back in about the middle of the receiver because the bolt was shortened and the receiver modified.  It's also a lower pressured round.