I am going to be casting some bullets for my 45-70 in both 405 and 500 grain. The 405 mold has a flat base but the 500 gr mold has a base that takes a gas check. The load will likely use a veggie wad between the bullet and powder and the bullet will be in the 8-9 BHN hardness range. Will I need the gas checks on the bullets if I keep the MV around the 1300-1400 fps range.

At that speed, I'd want gas checks.

The veggie should keep it from leading if it would at that velocity which I doubt.

personally if the bullet was designed for a gas check,
Id suggest using them, but obviously you could and most likely should
load a dozen or so with and a dozen without gas checks
and carefully bench rest test, compare the group size and barrel leading issues

I have been shooting lead bullets in all my .45-70 rifles and have never had any problems. I do use cookies in my antique gun loads. At these velocities I would not worry about it.