Mike,
I was curious about how people here in CA were getting away with their B230FTs in their 240s. Next stop: turbobricks! I just keep it all pre-smog.

Things you should expect to run in to:

Motor mounts, as you assessed.
B230 is longer than B20, I think. How much... I'd have to check.
Are you doing it Turbo?
Manifolds will have trouble clearing the hood on all sides.
Could lead to a custom front clip.
I've never seen an M47, M46 for sure you'll have to cut & widen the tunnel.
Read that M47s don't like being upright.
They came usually with M40s, sometimes M30s. Similar in size to M45.
Shortened driveshaft & different flange, bigger ujoints
I don't know if you want to stand the B230 upright... B21 & B23s I think people use the B20 oil pan & pump.
Custom exhaust
Disc Brake Swap
Electric fan
maybe aftermarket radiator

Only two things I can advise:
Learn Swedish or German, and peruse their boards for help & ideas. It's been done there without a doubt.

Rat Rods ethically should use the OEM stuff, period add-ons & your ingenuity to get the most out of it. & should start with a pretty tattered example of the car, so we folks don't cry.

I say get yourself a B16B M40 combo and make that work for power.
Or if it's late enough (post '61) a B18.

But I have a bent, so that's it neither here nor there. I'm making a period-style sleeper rally car out of my '70 142. Some guys I know make rat rods. My Citroen Wagon might be considered a Rat Rod if you want to get into the modifications it has. Mostly it just looks scary, and sits very low.

Just please don't chop it!

Don't forget the mexican blanket.

Have a blast,
-Sean
--
1966 122s, 1970 142s, 1974 142e... Blue is beautiful