I am sure that there are more listings where you can get a 5 main crank, these come to mind;
Scat, Molex, Crower, Velasco etc.
Crower I believe also makes the rods to with them.
Top quality rods are necessary when going this route.
Pistons are another piece of the puzzle.
IMHO, any OHV head that has siamese ports does not require a 5 main crank assy.
I have a 6 port flathead that has a 5 main, but is very very different than most.
John