Anyone have any insight on which heads to use on a 403 to boost compression? Have choices of 5A 68cc, 6 has &70cc and 7 has 69cc. Need information on what these heads came out on, and what they would cross over as far as part #. I know a 403 is not a good candidate for high horsepower, due to the bottom end just trying to get compression above 8.5-1. Thanks The #s are located on the left front and right rear of the heads.

 

A 403 moves as much air as a 400 big block. Why handicap it with small port heads from an SBO. Use BBO heads, port-match the intake as required, and either mill the heads or change the pistons to get the CR you want. The only reason to use SBO heads on a 403 is if you have a Trans Am and need to retain the stock intake so the shaker scoop still fits.

 

The heads you have are all around the same as far as flow rate. You can install larger valves if doing a valve job to increase the flow.

As for what they came on, all SBO engines used the same heads during any given year:
5 = 1968-1969
6 = 1970
7 = 1971
7A = 1972 (small size capital A)

 

The OP said "5A", which would be 1980-84 307 motors and not a desirable choice.

 

Consider 330 heads as well. #3 and #4 heads have a smaller combustion chamber than the 350 heads. Even #2 heads, if you can find them, will work however they'll have 3/8 stud holes at the rocker arms. They require the same work and most #3 and early #4 might require some clearance on the pushrod holes due to the different bank angle of early engines. Any standard replacement head gasket is also ~.025" thicker unless you get MLS adding another ~4cc of volume.

 

When using 330 heads you need to be careful if you’re going to install much larger valves. You’ll hit water if you take too much out of the bowl area.

 

I have two sets of #4 heads - One set is bare castings - Magged NO CRACKS - light rust. I can BB them in 10 mins and look better, but then I want more for them. . Plus, I have a set of take offs from a 330 complete.
I've ran #4's on 403's and my favorite head - other than a set of race port #5s or #6''s

$300 for the bare set and $250 for the take off's as IS. Plus the ride. Each head is about 47 lbs and will be roughly $100-$120 to ship.
Each head takes about 1hour to wrap and single box. PITA ! I hate shipping heads.

If you want/need the 330 forged crank out of the same motor I pulled heads - its available as well to. Bigger PITA to ship. NO Flex plates....
My zip is 78681.

Email is sigtauswt@yahoo.com