IIRC it was a 1/8 thick ring. Ill have to check as I think I got both in the garage...
OK here are the specs: 65 273 register 1.550 68 convertor snout 1.808
so you need ~.129 thick bushing or 1/8 with a little wiggle room, maybe knurled. Id imagine 1/2 deep would be fine
I dont think it needs to be an interference fit but those specs are pretty close with a dial caliper. Suppose brass would be good as its easy to machine, or whatever he has. AL may gall.