But if I find that I'm having to put a lot of effort into getting it to shoot even marginally well, it's very likely that it's going to be bored out and rechambered to .38-55.
I think your best bet then, would be to concentrate on bullet nose fit. If the bullet nose falls in under its own weight, it is too loose. If you have difficutly or it will not enter the muzzel at all, it is too tight. If it will go in with resistance to thumb pressure, it is just right. My favorite and best shooting bullet in my 30-06 was a Saeco 301. But that bullet nose was way too loose in my 30-30...had to find one with a larger diameter nose.
If you can get some samples of the bullet you are considering before buying and see how the noses fit the muzzle (slight press fit), of your rifle, and you find a bullet that has that press fit, it most likely would result in acceptable accuracy.
Being that you wish to buy your bullets instead of casting them yourself, indexing them is out of the question anyway.
A single-shot, bolt action 30-30 re bored to .38-55 would be disconcerting to Cast Lead Bullet Rifle Shooters, inasmuch as they seek them out for target competitions and guns could not be re-bored and still shoot in the Factory Sporter class. A re-bore would represent a yet smaller pool of 30-30's from which to seek a good Cast Lead rifle shooter.