[spdbrake]

I take it your building a pan.

You could use a rigid gasket like this to pull measurements from.
JEGS Performance Products 210298: One-Piece Oil Pan Gasket 1969-96 Ford 351W/5.8L | JEGS
or
Ford Performance M-6710-A351: Oil Pan Gasket 351W/5.8L Block | JEGS