....why would you possibly use straight 30 in that thing. Unless you're pulling a disc or a bush hog, there is no reason to do that, ever.
Drain it, use regular 10w30 Valvoline, or the HM stuff to condition the 40+ year old seals. It'll be fine. Had the same thing happen with a rebuilt 440. Used M1 10w30, leak city. Drained it, put in 10w30 regular old white bottle Valvoline....problem solved.
Keep in mind what these things used to run in 40 years ago....they will run on anything. You could pump crude out of the ground and refine it with a window screen and it would be fine.