Based off the picture and the tight clearance in there,, I don't see that you have much choice but to grind it.
Where did you buy it from? Perhaps they can exchange it,, but realize that for it to fit in there, it'll be ground to fit.
I started grinding. Currently removing stock from the bolt. Looks like 3/8" is what I'll have with mine as well. I really think this should have been better designed. I have three full threads left on the bolt.
My 3 point will be attached there as well as the back of the transaxle. I over engineer everything. Therefore, I'm not to worried. Thanks for the link. I have a better idea now.