I have the 68 #16 heads on my 73 400 block. Got front end rebuilt so I can safely drive it now. I took it out last night. It tends to run hot in stop and go traffic, but fine on the road. It also had preignition problems when I shut it down, I have installed an HEI type ignition system and new Boesch platinum 4208 plugs gapped at .035 I have the timing set at 11B. I read the earlier posts and intend to check my water pump clearance. Does the preignition have to do with the running hot around 200 in traffic or is it a seperate issue? Thanks Louie