Bismuth is safe in old guns, it won't damange choke or barrel.
Might be kind of hard to find specific loads this season, the company that imported it has gone out of business.
But there's still a ton out there, just call around to the various suppliers and see what they've still got in hand.
As far as pressures I don't know, anything in 1 1/8 ounces of shot should be dandy. I've used the 1 1/4 ounce loads in a Parker with good success.
Shot Kent TM 1 1/4 ounce #5 yesterday for the Ontario duck opener and killed my 6 ducks in 8 shots I believe. That's good stuff but the boys on here seem afraid of it for some reason. I wasn't using a Parker this time though, was shooting a 1905 vintage Remington Autoloading shotgun.