Which bellhousing would be best

Ben Drinkin said

I believe your input bearing retainers will be different sizes, therefore different size 'hole' in the bellhousing. I think pick the bell that 'fits' the retainer of the trans you're going to use..

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The challenge is that the iron bellhousing has a different shape to it because of it needing to fit in a smaller tunnel. And the over drive housing does have the larger diameter hole but, does that really matter if the hole is a bit bigger that needed? does it need to be a exact fit?