Anyone know if 8.5mm was the factory spark plug wire size on a 1970 440?

Definitely not. They were pretty small to fit in the stock wire looms. Seems smaller than 7mm.

I think it's 7mm but it might actually be a little smaller

Back in the day, 7mm was the first step up in size for performance plug wires; 8mm came later, larger sizes later still.
Can't recall measuring OEM wires back then, but I'd guess they were smaller than 7mm, and likely an SAE dimension. They seem in retrospect to have been pretty small and not as pliable as what we have now.

Does anyone make 440 wire looms to fit 8.5mm wires or do I have to modify factory looms?