No custom bellhousing needed - the bellhousing pattern is the same between a B18/20 and a B21/23/230. You could bolt the B230 right up to the 1800's transmission and bellhousing. You'd just need to
- swap oil pans and pump pickups
- probably a custom intake manifold - since there'd be a lot less room on that side

On the plus side, it might make more room on the exhaust side - perhaps enough to use a stock exhaust manifold and still have room for the turbo.

Motor mounts, however, would have to be fabricated, upright or slanted installation. I think Yoshifab makes some adapters that let you bolt the old style B18/20 motor mount brackets up to a B21/23/230 block.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)