I have a 1981 version of the 760 pump-gun. I have to small-base size my reloads. I would have feeding problems very rarely but, did not want to take a chance of my gun hanging-up at a critical moment. Most factory rounds are even "narrower" than your small-base dies will size them. However, if you choose to purchase the small-base die, you can pretty much forget about feeding problems related to fat cartridges. If you run your reloads through it and, it consistantly feeds them flawlessly, you may be OK to full-length size only.
I would trim all brass to the same length, size, and then determine the optimal overall length to put the ogive a couple of thousandths off the lands. Take some time and put in the effort to build some top-notch ammo for that wonderful old rifle! Then go kill something.
I hope you enjoy that fine old 760 as much as I do mine.