i have a nice std bore c5ae-a block with bosses and thick main webbing

By chance have you seen this post in private classifieds?

For historical purposes, what is the casting date on your block?  Also, it sounds like Kevin would be interested in a picture of the C5AE-A casting on the block for information gathering he has been doing.

       Yes, 35-45 p.s.i. is about as high as most shops will go. 

       First, as mentioned, the typical "pressure-tester" set-up and particularly the manor in which the exposed passages are blocked won't generally hold beyond such, if even. 

       Next, be careful (always) if running the pressure to higher numbers, as when you consider "area times pressure" there can be a significant sum of force being retained!  Can you picture a "core/cup-plug" coming dislodged from the side of the block and the potential velocity it might possess?   

       Scott.

I generally do 40 psi if I am doing one with freeze plugs.  However, I have run it up to line pressure on a couple of Genesis blocks. 

FWIW, I do mine with a torque plate and a water pump fitting, one side at a time.  I say that because mine is not a pressure tester, which generally use clamps and rubber gaskets with plexiglass or alum plates, so those certainly run lower.

I'll tell you, at 40 they bubble very clearly with soap spray.

I did my own and can be seen in my rebuild thread here;

https://fepower.net/simplemachinesforum/index.php?topic=7720.0

Hands down one of the best modifications I have done. The car has never really been set up to handle with bigs and littles etc. but this has made so much more difference to the confidence I have putting it into corners. It is predictable, more stable and feels more planted. I would venture to say that before anybody considers suspension upgrades on a unibody Ford (especially pillarless coupes) they should install chassis connectors. It made that much difference.

Our Mustang and Cougar both have this style of cross brace on them. I looked and they don't seem to sell it for Fairlanes and Comets.
https://totalcontrolproducts.com/p-1612-mustang-64-70-cougar-67-70-hardtop-g-connector-center-support-standard-torque-arm.html

We have experience with this style of X brace and can vouch for this type of subframe connector being beneficial. The cars are much more solid with it installed. The middle X un-bolts. I have driven the Mustang without the middle X in it and the passenger door window will pop out of the seal if you make a hard pull with the car.