RHS or EDDY

Y ou need to know what the j heads flow now, otherwise in a ootb scenario ...you might end up with the same flow and only more money spent.
The closed chamber will bump up th comp a bit, but probably not much more or beyond the limit of what the current heads can deal with on pump gas.

I've run 9.7 - 10.1 comp with open chamber j's w/no issue and on pump gas 91

the other thing is....that cam is not a good choice for the 408 or much else anymore, and if you go some 1300 heads that flow more 'but at a higher lift'...you wasting ur time only lifting the valve .480....and when in reality you might actually see a whole .450-.460 lift due to lifter angle/push rod angularity.

either go with a bigger cam, or flow and port what you have to work better.
You can get the same big low lift numbers from a j head as you can an rhs head
remember most of these replacement heads are just that..