First step, clean that bore with brake cleaner on patches. Lyman uses some kind of packing grease that's just about impervious to any other solvent. And until you get it all out of there, the bore is going to be "tight" with almost anything.
There's also the factor of "shooting in" the bore. It's going to start off liking one combo, but after 100 or 200 shots to smooth it out, odds are that it's going to take a little bigger patch/ball combo for accuracy than you started with. I'm betting that in the end you'll be really happy with the .530 ball and ticking, but you might have to start with .530 and a .015 patch.
In either case, go with the .530. I bought and tried both, and the .535 required me to use .010 patches, and even then caused frustration. The crown on the GPR is sharp, and it was next to impossible to get a .010 patch started without cutting or tearing it.
Even with the .530 and .015 or ticking, go easy on starting your ball. If you "smack" the short starter, you'll cut the patch almost every time. Better is a push, slow and steadily adding pressure till the ball seats. And watch that you don't get any folds in the patch. Another sure way for cuts.
I wouldn't do it first thing unless you already have to tool in hand, but you will be happiest with your GPR if you cone the muzzle. The tool is cheap and it's simple hand work. You won't even need a short starter when you're through, and there's nothing better when you're cutting your patches at the muzzle.
None of what I've said should be taken as criticism of the GPR. I love both of mine (a 50 and a 54), so I'm just passing on practical experience and fine tuning. The results are very much worth the fiddling!