Just got myself a 68 Barracuda Fastback!

68cuder said

Yea that'd be super cool but my trans is a 23 spline and if I'm not mistaken it will only fit a small block. Plus I'm planning on using some of the sb accessories and brackets for the 408. And my bellhousing. But yeah a big block would look amazing in there.

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Your trans will work behind a big block, you just would need a different front case with larger diameter front bearing retainer or an adapter ring. I ran mine decades ago behind a small block.

Seriously, the only thing that matters is getting and keeping it on the road. The best way to go is whichever way will get you back together so you can enjoy it. You don't want a garage queen like mine; under construction for over a decade now.