If it is a 400CI with "670" heads, it is the 14 degree
valves, screw-in rocker studs, and big valves. The early 400CI in '67 that used the "143" head still used the 20 degree
valves. These would have been found on the full size cars and had the smaller valves. These heads were then replaced sometime in mid year-ish with the "061" head that had the 14 degree valves & the larger valves. Both big car heads used press-in studs.
However, the pistons appear to be the Sealed Power 389 cast pistons in matching them up. Take a look here: https://www.summitracing.com/parts/s...0/applications
Pontiac 400CI pistons have different valve notches in the piston tops to accommodate the larger valves which were moved inward towards the centerline of the cylinder bores along with the 14 degree angle of the valve. Here is a Sealed Power 400CI piston top: https://www.jegs.com/i/Sealed-Power/...yABEgJXdvD_BwE
Realize you are going with the aluminum heads, just get the correct 400CI pistons for your build and you will be OK.
The following is just out of curiosity - you are 100% sure it is a 400CI? Now if you have the "061" heads with big valves, they are very good heads to work with.
My opinion - I have not seen what an early 400CI piston top looks like using the "143" 20 degree heads, but assume the piston may have resembled the 389 valve notching. However, it is very possible that 389CI .060" (which would be the standard bore size of a 400CI), or even .090" (400CI .030" over) pistons were used to match the earlier 400 "143"
heads OR you have 389CI heads on your 400 block and the pistons were used so they could be used. What throws this out the window is that you said you have the large valve heads so............what heads did your 400CI have?