What is the best Diameter bullet to slug the bore of a 40-70 Ballard? I’ve got a rifle soon to be on the way and am looking to get the right size bullets. Any other tips and suggestions are greatly appreciated.

Try egg sinkers - get 'em at Walmart. Nice and soft, and in several sizes.

A .40-70 Ballard bore will usually run around .410"-.411" and I use muzzleloader balls. I have two boxes in .45" and .375" and use the appropriate size to slug bores. 
For bores a bit larger I lay a ball on the flat of my vise and give it a hit with a hammer to squish it. Then measure with my mic to see if it's large enough. Once it expands to cover the size needed I sit it on the muzzle and use a brass hammer to start it, then a brass punch to tap it into the bore. Once it's tapped in below the crown I use a cleaning rod to tap it out and then mic it. Going all the way through is usually a waste of time, as bores generally get larger the closer they are to the chamber. A chamber cast is best forr checking the chamber end bore.

Hopefully you have a source for donor brass like .405 Hornady brass? The only real option is .30-40 Krag and a lot of case stretching after fire forming to get to .40-63/70 Ballard length.

Marlinguy-good info!  I always where people got the balls to check bore/groove.