cam size is dictated by rpm and if you have the heads that will work for that rpm the size of the engine the intake the cc on the intake side of the heads and the comp is what it takes the cam to work rigth at the rpm .you have a small engine 427 with big heads you will have to run this hard to get it to make the same hp as a 454 or 496 the bigger engine will make more torque at lower rpms and the rpm range say 3000 to 7200 +dose not mean you will have to run at 7200+ to get to work but the bigger cam in a small engine will have to be runed a lot harder when i build a engine i think what dose this thing have to do?? how much comp can i run in this dose the customer want to run race gas??is it in a lite car or truck if raced how long is the track what intake will it have. a stick or auto trans in it . do they have parts they want to use . this some times can not the best comb on big block i try to get the biggest engine they can afford and not run it as hard and it will not need so much comp to get it to run but a bigger engine can take more cam so you need to know what size big block are you goning to build 427 or 526 ?? the heads you have would work best with a bigger engine as for just picking out a cam out jeg.s you need a cam company books try to fined some that used their cams .but getting the rigth cam is not all ways easy i have used comp lunati crower crane howards isky with good luck you need cam books from more than one to lookat to pick a good cam and if you cant find one you will need a custom cam$$