Lots of original manifolds around here, on boats much older than yours. To put in new gaskets where the manifold meets the block I would separate the two completely and clean the surfaces really well to make sure there's no old gasket material stuck on there. Same if you were to separate the riser; I took everything apart a couple winters ago and flushed out thoroughly, then cleaned the surfaces and used new gaskets to reassemble.
But (as wotan said) you might be able just to slide the old manifold-to-block-gasket out and put a new one in; then tighten everything up and if you still have a leak, then go the extra mile.
If you listen to older (90's, early 2000's) Fords, especially those with 351's, lots of them have manifold leaks, at least up here in the salty Northeast.
1989 Bravura - repowered with 2001 PCM 351 GT-40 - 1.23:1 PCM Trans.
1989 comp - sold
1985 comp - sold