Cam grind has everything to do with octane requirements. My 331 requires at least 92 octane with 10.4 to 1 and the Ford Z303 cam. The last 390 I had needed 93 with the stock cam, 89 with a Crane 272* Energizer.
With the stock (dished) pistons in a '74 302, your compression with those 289 heads will not exceed 9:1. It's 8:1 with the stock 302 heads. The 268H will be fine. My friend runs around 9.5:1 in his 302 with the 268H, '66 289 heads (54cc chambers) and flat-top pistons. The block is not decked. He gets absolutely no detonation with regular gas, and 38 degrees total timing.
I checked the heads again and I'm sure there just plain heads with screw in studs and aftermarket roller rockers... would my E5 heads work better with the 268H cam ? they are in great shape to...
The E5 will lower the compression but will have hardened valve seats which is a plus if you are going to drive it daily...
Find out exactly what pistons are in there before making a decision on a cam or heads. I would stick with the 289 heads if not going to drive it everyday. You can run lead substitute to mitigate any valve seat wear. If you know a professional mechanic, ask if he's got a bore scope, if he does, then it's a simple matter to pull one spark plug and use the bore scope to see what the piston tops are (flat top or dished) But even with flat tops. the compression may be less than what you'd think, depending on the pin height.
The pistons are factory original, standard bore.. I had the cylinders honed and all new bearings installed,new rings... Crank and rods are also original too... I had this motor refreshed in 2006 when it was still in my 74.. to this day it has 72 pounds of oil pressure at start up and 65 pounds warmed up..Temp runs between 160* to 179* all day long... It has been a great little 302. Someday I will install a RC107 and a stroked 351w but until then this car will just be a driver... going to add a/c this year too..
If they're the original pistons, then keep the 289 heads where they are. Putting the E5's on will kill the comp ratio, putting it down in the 7's.
Unless you already have the cam, I'd recommend a dual-pattern spec. You'll get more torque that way, as a higher lift and longer duration on the exhaust side compensates for the poor exhaust flow of those heads.
