I have been through this before but only because a prior owner did not install it. This was in an automatic but same principal. My opinion is that you probably can get away with out spacing it. The spacer would move the transmission back but will not change where the bendix engages the flywheel. The flywheel locates off the crank. Spacer or not the starter will be flush with the plane of the rear of the block as long as the transmission mates to the rear of the block as it should.  As long as there are not interference issues I think it would be fine. BUT if you can make one or use washers then do it.
 
HOWEVER, like others mentioned the plate does locate the starter. Not having it located will cause issues. I found out the error when a starter went bad and we replaced it. Then it would not engage. This was the second starter for the truck, when I bought it we had to buy one and put it on and somehow my buddy got it right on the money(unaware/not mechanical) and it worked for two years. When I did the second one my luck did not pan out so well. I
guess we all have those friends that a rabbits foot! Mine got burned up a long time ago!

The FIX, my old man(bless his soul) was a tool maker: I will never forget this: He said: " If damned Ford engineers did their job right, everything should located and center off those three holes" so he scratched his head and came up with an idea.  The locating issue comes from the starter housing bolt holes. They are "loose". Everything else the bolts go into will center/tighten. So what we did was find some bushings. Tight on the hole in the housing and tight on the bolt. By doing this it brings everything into line/center.  It worked like a charm and lasted until I blew the motor a few years later.

It is not perfect but it beats pulling an engine out. He had the bushings so no help in telling you where you can get them. Just trying to throw another idea out there for you.