Powders that work ideally in a given bolt action rifle might damage an M1.
Hmm, let's see...so this is a 308, and
a tanker. That complicates things. First you might want to ask the guy who did the tanker modification what he recommends for ammo and powder, if you know who did the modification. I'm not sure that standard M1 Garand advice can transfer 1 for 1, here. The barrel length is different so the gas port is going to be a lot closer to the chamber, here (the gas port on a standard M1 is quite close to the muzzle). He might have also played around with the gas port size. All of this could affect function, and how much pressure the gas piston "sees."
But without intimate knowledge of what was done, for the powder not too fast, not too slow is likely still good advice. But drilling down...if you look at M2 Ball, the .30-06 ammo that the M1 Garand was finally designed around, yeah, sure, IMR4895 is the ticket for reproducing M2 Ball loads, with IMR4064 being the sure ticket if using heavier match bullet loads (such as for the Sierra Matchkings).
On the other hand if you look at M80 Ball loads for the 7.62X51NATO, the powder it was designed around was WC846:
The commercial equivalent of this is BL-C(2). If you look at a burn rate chart:
Your Varget or R15 actually look like they might be better choices than IMR4895 if
the gunsmith who modified this M1 designed the thing around M80 ammo, particularly
if you start using the heavier 168gr bullets that you mentioned. If your powder is too fast for the bullet weight, you can start putting some improper bends in that op rod pretty easily (the op-rod is supposed to be bent in a very specific way), and your accuracy will go to pot, and you'll have to get the op-rod rebent to spec to get it working right again. The adjustable gas plug will give you a little wiggle room, though. Just follow the installation instructions...turn it all the way down such that the action doesn't cycle, then gradually turn it up until you have reliable cycling with your ammo of choice, and no more.
All that said, it isn't completely inconceivable that your rifle could love IMR4895 charged handloads.
Hard to say for sure what the gunsmith in question did to this rifle, though. There are a number of folks that do this mod. Hopefully it was one of the more skilled outfits that did it such as DGR, instead of a chop-job in Bubba's shed out back. An improperly setup and fed gas operated rifle is nothing to play around with...not worth risking getting maimed or killed over. Incidentally I wouldn't mess with a .308 M1 Garand, myself. Well, at least one that didn't have a carefully calculated reduction in the mainspring spring rate. It's a long-action autoloader...giving the bolt free time to accelerate the cartridge beyond what it was designed for by running short action cartridges through it without modification is just asking for troubles with slamfires and such.
You may or may not take what I say with a grain of salt. I'm certifiable OCD about firearm safety and keeping my boyish good looks, and loathe the idea of living with unnecessary design compromises or shortcomings in my firearms when I don't have to.