MustangTek claims those heads are 64 cc but since they are on the shelf and especially since they have been worked on, I would cc at least one chamber per head.  Cheap and easy.  You can then plug all the specifics into an online CR calculator (e.g. Wallace Racing) and see where you fall.

Pat

https://www.mustangtek.com/heads/Heads.html

Also I think the inner core will only pull out from the speedometer end, not the transmission end, because of a crimped bead on the top end of the core wire.

Pat

More information about the engine/vehicle/situation would help.  Do you have an automatic transmission or power brakes?  Those both have vacuum lines that can leak.  Usually intake manifolds don't develop leaks after years of being fine.  Can happen I guess but not the norm.  Was the intake recently removed/reinstalled?  Any other work recently done to the engine for that matter?  Often things like this can start after working on something else (e.g. new condensers are often not of great quality).  What kind of carburetor and ignition (i.e. points, duraspark, etc.)?  You might check the spark quality by pulling the center wire from the distributor and having someone crank the engine while holding the end of the wire near the engine block.  Use gloves and insulated pliers, and it's easier to see indoors or when dark.  Should have a good blue spark.

Pat

Edit: Sorry - you mentioned in park, so it has an AT.  Did you block the line going to the vacuum modulator on the transmission in your earlier testing?

What is the timing at idle?  If you can bump it up some, it would allow the throttle plates to be closed more, which might make the mixture screws more responsive.

Pat

What kind of heads are on it?  More specifically, how many header bolts per head?  If it's the plain vanilla type with 8 bolts per head, I would definitely shoot for getting them all in before any leaks develop.  Others will probably have suggestions on bolt type and brands that can make the job easier (12 point or 5/16", etc).

Pat

I don't have direct experience either, but I've read that "E-85" can be "up to" 85% alcohol but supposedly it varies quite a bit from station to station and maybe even batch to batch.  E-85 compatible vehicles are able to sense the burn rate (or whatever) and automatically adjust the A/F mixture but with a carburetor, it seems like you'd have to be be much more certain of what you were running.  Might be doable for a race only situation?

Pat

Does it overheat at idle or cruising or both?  What is timing, and can you increase it without detonation?  Small water pump pulley is a good idea.

Pat

I've used DigiKey for components for electronic pinballs (transistors, capacitors, resistors, etc.) but it's been awhile.  If there is a "Micro Center" store near you, they have a selection of parts like this as well.  Similar to Frys, which closed around here at least.  Big store but they have a couple of aisles of electronic stuff and handy if you have one in your town and don't want to wait on delivery.  I've heard Hobby Lobby has a small selection, too.  Maybe even sells Radio Shack stuff now?

Even back in the 1990s and 2000s, when Radio Shack was still around, every time I went in there, it was hard to get someone to ring up a purchase because all the clerks were busy selling phone plans.

Pat