5.0 Engine Trouble!!!

I recently installed a 5.0 roller motor from an 86' SCCA car in my 67 notchback. The problem is in the cooling system. I have a 2 row radiator and stock fan and shroud from a fox body. The radiator cap and thermostat are brand new and she's got a jeg's overflow tank.

When the car is started, the temp gague will creep up slowly, like over 15 min to overheating. The strange thing is that the heater in the car will be ice cold. Then, if the pressure (steam) is released, the car will run in the middle temperature range and the heater will blow moderately warm heat.

Do I have an air pocket in my engine or should I be thinking about a head gasket replacement? I've checked the plugs and they all look to be ok. And I know the next step is to compression check, and that's going to happen asap.

Any thoughts??? Thanks for your help
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The really bling way to fill your cooling system is to start with it empty, then use a air operated vacuum pump to put the system under 20" or so of vacuum, then use the hose thats attached to the pump, open the valve and suck the coolant in. This is how I did it when I was working for Ford and Daimler/Chrysler building prototype trucks. It works great and you never have to burp the system.
The serpentine belt engines use a reverse rotation pump, not reverse flow. Latemodel Chevies have reverse flow.

Both the timing cover and water pump are different.

Yes, you're correct. A poor choice of words. The question remains about the mix and match. It's more common for a parts guy to sell a reverse rotation pump to someone with an earlier block, and non-serpentine belt drive. One other idea which comes to mind is the thermostat (provided it's not upside down). It's easy enough to check whether it's defective. Get a thermometer and place it in a pot of water on the stove.
the accessories are all original from the 86. i just used a canton pan so that i would have a dipstick, so no trouble with the waterpump.

the only way that it will run without overheating is if i open the pressure release on the radiator cap. otherwise it will build up an unreasonable amount of pressure and overheat. she runs at 1/2 temp but no heat whatsoever
I'd have to agree with drilling the t'stat. I've done it to several thermostats, stock Ford ones usually have the "jiggler" to release the air, but aftermarket ones usually don't. I drill a 1/16" hole in the t'stat, clock it towards the top, and it works everytime.
accessories may be all 86 but make sure you have the belt turning the pump the right way i spent weeks and alot of money replacing a whole cooling system just to find out my water pump was turning the wrong way,its easy to do i am sure you have change some things like no smog pump,a/c its easy to place the belt on the wrong way after modifying the orignal set up