I would have sent the converter back and had it fixed
To late now
I just went through this on a Powerglide to a RB motor, the old motor, different owner, had a 6 bolt crank in it and mine has a 8 bolt crank, the 9 inch Powerglide race converter drug on the flexplate, no end play at all
I could have had the adapter between the crank and the flexplate machined down but I ended up buying spacers, both 1/16 and a 1/8, from Mikes Transmissions in Palmdale, CA. I used the 1/8 spacer and it has right at .120 end play now, just what Mike recommeneded
BTW, I've bought and used a bunch of different brands and sizes, 12,11,10,9 and 8 inch race converters, 727 converters, never had a problem like yours