Intake manifold fitment

The pic of your engine shows it has 'LA' valve covers since each valve cover has 5 bolts. Magnum engine valve covers have 10 bolts.
But that isn't decisive since you can use 'LA' valve covers on Magnum heads by only utilizing 5 bolts.
The common way to build a 408 stroker is to start with a 360 - either a 'LA' 360 or the later Magnum 360. You will need to dig inside to find out what it is.
If you are considering a change from carbs to EFI, you should also determine your existing cam specs. EFI is happier with fairly mild cams.