Surprize surprize, cooling issues

[67FEfastback]

Thanks guys, a lot of helpful info in your replies.

Yes - new motor, standard stroke, aluminum heads, 040 over but lots of thickness on the sonic test.
Standard pullies

I am going to try and water down the antifreeze. I am certain when I replaced the thermostat, that I am over the 50/50 mix.

The timing is not too advanced, It may be retarded though, but I cannot confirm, without a timing light. I am concerned that the vacuum advance is too much, though (MSD 1 wire dizzy)

I checked the radiator. I believe it is a standard 3 row radiator, not the HD 4 row, I'll have to suck some antifreeze out the top to count the rows.