Years ago I rebarreled a 1903 to .35rem. I stayed with a similar case size but with the obviously larger neck diameter. I machined the spool to accomidate and it functioned well.

On your proposal, the 6.5RWS should ride the spool well, with the neck area being somewhat smaller, may well work without any alteration.

I have not measured a 6.5x65mm case so can only go by spec sheets. It is stated as being larger in diameter at the base....???.....Possibly, possibly not.

I think I would procure some ammunition and try it thru the spool. looking at the specs, other than the .003" larger base, it should chamber fine. The body is longer, but the shoulder is smaller.

Sounds like a very worthy project.