Gentlemen. I want to build a mold for a .70 bullet to shoot as cast. Do I need to adjust mold size to allow for shrinkage on something this big?

Shrunken Bullet Syndrome is not easy to predict.
Sometimes enlarging the sprue hole is all that is nesseary sometimes not.
Over at the 12 ga from hell posts, on the last page, there is a very large mold with special vents.
Give it a look

John

You might take a look at the 12 ga moulds that are already on the market. The Lyman and Lee moulds are made to fit inside of the shot cup and are very close to .700". You could Beagle them to adjust the size a little.

www.mountainmolds.com used to make shotgun moulds. I don't know how busy he is, but it might be worth looking in to.