I think I've seen dimensions of the plates on a merc parts website. Dont remember where though. BTW, they have both side by side and staggered, make sure you get the right one.
But if they're too big, just cut them off. If too small use 6 x 6 L angle.
As for the rear mount, you usually buy a new bell hosing so the starter (top mounted) and tranny input flange seat fully. Or you can mill the plate thickness off the front of your bell housing. But the only reason I can see to use the rear plate is for a staggered motor using a driveline output. Even SSM drives have tranny tailstocks that ride on the transom plate.
If you want to use a bottom mounted starter, just cut a hole in the plate.
Gary

Thanks Gary. Lots of helpful info there.

Found the diagram with dimensions.
Side by side:
http://www.dolleroffshore.com/parts....pn=4269&r=4440

Staggered:
http://www.dolleroffshore.com/parts....pn=4269&r=4441

It looks like I only have about 21" between the stringers. It also looks like I can take the base Merc front mounts, cut to size and add some strengthening.

I'll let everyone know how it goes!

FYI, if you get 'stuck' give me a shout. I'd be happy to help with any custom needs. Especially the unique stuff.

Sincerely,
Brian Kamrath