The antifreeze in the cyl's could be coming from the intake, that's true. It could also be from a blown head gasket. See if there's antifreeze in the oil, if so it's a sure bet either the intake is leaking or head gasket blown, or both. If you've no (or very minimal) coolant in the oil, I'd be more inclined to think maybe you've got head gasket issues.

The 30 psig less compression in #6 could be due to head gasket however could also be due to a burnt exhaust valve so you may have more issues. You probably won't be able to tell much until you tear into it.

At this point, if might not be a bad idea to pull the intake then both heads, with what you've got to pull off to get at the intake, the heads aren't that much more.

I wouldn't be surprised if the shop that did the intake R&R cut the new manifold gaskets instead of doing it the right way, and loosening all the rocker arms so you could pull the pushrods and do a clean install on the gaskets. The "lazy man's" way is to cut the intake gaskets so they'll go around the pushrods. You can get away with this if you do it right, but there's also a lot of potential to mess it up.

Check the gaskets out carefully when you pull them, for signs of a botched installation.

G'luck and let us know what you find..........ed