Have you looked for a standard .45 pistol bullet mold that may be 255,265, 285, 290, or 300 grains to use in that rifle? Lyman and NEI and others have all kinds of molkds for heavy pistol bullets out now, and its just a matter of sizing them down to the diamter you need in the rifle. A couple of the bullet companies sell Sized and lubricated 300 grain .45 caliber bullets also. Take a look through Brownell's catalogue, for instance, to see what is offered. I think you should be able to find a bullet that works in any .45 caliber barrel, provided its sized down. The cost of a sizing die the correct diameter would be worth the expense.
You might even want to look at .44 magnum bullets, cast lead, of course, that come in .429 diameter, and then paper patch them to fit your .45. For info on paper patching, see:
http://members.shaw.ca/bobschewe/