I've got 1/4" spacers on mine also and that usually coincides with the thickness of the mid plate. The converter I have also has different length removable pilot snouts that fit into the crank hub as you MUST make sure you have enough of that in the hole to keep everything concentric.
Another thing too is make sure that the rear block dowel pins are long enough to reach through the mid plate and into the alignment holes in the tranny case.

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Big Boyz Toyz!
Necessity is the plea for every infringement of human freedom. It is the argument of tyrants; it is the creed of slaves.
William Pitt, British Prime-Minister (1759-1806)