340X heads vs late model LA heads vs Magnum.

IMO any gains you "might" see will not be worth all the money and labor.

Labor is free as I will be doing almost all the work myself. If I use the heads I have either the 318 swirl port heads or my 340x heads the I am out the cost of valve springs and gaskets. More on the 318 heads as I will need larger valves and seats installed.

I plan to update the intake either way.

Does anyone have a way to check valve spring tension with the heads still on the car?