my 408 build

That is the same basic build I am doing. If you read the comments of others, a lot of them think the cam is too small and the J heads will hold it back. Although I think 12 second car with that combo is very doable.

I bought the cam with the complete Comp cams kit (K-231-4) and found out they include valve springs that require a higher installed height than what is available with stock mopar valves in the J heads. Comp Cams 986 springs came with kit have an installed height 1.75" , stock heads are good for 1.65" or so. Sort of Pissed me off, that they sell a cam kit for a mopar engine with the wrong springs in it!!

I went with a set of MP springs (P3412068 8) cause I had a spare set from my present engine that were basically unused). You could also go with a set of Comp cams 994 or 995 springs (mancinin sells the 995's ) which will work with the lower installed height, allow you plenty of lift before coil bind and have a slightly higher spring rate (not sure how that will effect cam wear or overall performance?!).