The only 4our I have played with was a 438, but I assume yours also has the intakes in the "heads", and the exhausts in the castiron "cylinders"??
I believe the exhausts were seated directly upon the matrix iron, and installing a seat insert would merely require cutting an appropriate counterbore. Soft castiron seats need only a couple of thousandths interference (.002"), since they will have the same expansion coefficient as the cylinder. Hardened inserts(necessary?) would require slightly more (.0025"-.003"), as they tend to shrink if overheated.
"Clinching", or rolling the edge of the counterbore over to retain the seat insert is always adviseable.
The decisions you must make are:
Are the seats really that sunken as to need replacement?
Will the machine see such service as to require hardened seats?
Availabiltiy of seats specifically for this application are probably available from Max Bubeck or Dennis Young, but they are most likely to be a Martin-Wells offering for a modern vehicle that has been cut to size. With H-D's, it is very common to custom-cut nearly every seat OD different, in order to save matrix for the next installation. K.O.Lee even offers un-finished blanks for this reason. And lathe-cutting the OD and depth of the insert AFTER cutting the appropriate counterbore insures a proper fit, as the counterbore is the difficult cut to be accurate with. And an insert is a lot cheaper to scrap than a 4our head or cylinder!
Conducting heat away from a hardened seat is critical. Silver Seal Corp. makes "Seal-Lock", a white goober that not only chemically welds the seat in, but fills the insulating voids. Good stuff.
Or you can let Wilson Plank oven sweat and silver-solder them in!