My shop manuals for my 427 Galaxie list cooling capacity at 20 quarts with the heater in the system . Subtract 1 quart for no heater . My gut feeling is that you have air in the system and need to bleed it . Try jacking the front of the car up , crack the upper right hand fitting on the radiator with some pressure on the system ( you said you have a pressure tester ) until there are no air bubbles coming out . Air pockets WILL cause the temps to spike . Did you drill two 1/8 " holes in the thermostat at the 12 o`clock position ? This really helps bleed the system . Also , remove the temp. sensor from your intake until coolant comes out with no air bubbles . Messy ? Yes , that`s why I do my initial startups on pure distilled water instead of coolant .
Stupid question and no insult intended , but is the thermostat in backwards ? I did that once .
13 lb cap is fine ... mine is 12 psi and I run 180 degrees . The ERA fans should keep you in that range in the garage as you aren`t in the really high temps yet .... though we will reach 90 here today . My ERA 482 never gets above 180 in the garage using only the ERA supplied fans .
Also , if your timing is retarded to much , the engine will run hot .... I run 21 degrees at 1000 rpm and 37 degrees max per my engine builder , Robert Pond .
Are you running the MSD distributor or another as that affects the wiring ??
Good luck ,
Bob