Turbos depend more on their sizing relative to the engine displacement to determine where they make the bulk of their power. Large turbo-diesel engines in semi trucks do not rev high, yet the turbocharger is perfectly sized for their narrow rpm band.
A Garrett T03 turbo, which is both water and oil cooled and is commonly found on turbo 2.2 liter Dodges, would be a good size for a 235. In a 135 cubic inch engine, the T03 begins to get strong at about 2800 rpm. On an engine with 100 more cubic inches, that'll push the powerband further down. You'll probably start pulling at around 2000 rpm, which is also about where the 235's torque peaks.
With the right cam, you could probably do 4000 rpm on a 235. I usually don't go much past 3600 rpm on mine. You can go higher, but you have to do some pretty significant building up to get there. Look at Leo Sanducci's site and check out how he built up his turbocharged inline 6. You could do something similar. For example, if the block will support a 0.173" overbore, you can use forged small block Chevy 305 pistons. I don't know the compression height comparison, but you could probably get big block Chevy rods for it, which come in a variety of lengths. The big end of the rods can fit on a 235 crank turned to that size and rebalanced. Then a good cam, and you could easily see 5000 rpm.
If you do that, then you probably want to up the size of the turbo to match. Like maybe a Garrett T04, or even a Super 60 maybe.
The heads don't flow the best, but the turbo will force feed it. I'll bet you can make your own fuel injection system using a V6 EFI system from a salvage yard. Or just put a hat on a carb. I'd probably use the head with a good valve job to improve flow. Leo Sanducci cut one cylinder each off a pair of AFR aluminum SBC heads and welded them to make a single inline cross-flow head. Very trick but very expensive.
I'd like to turbocharge mine. I have a few extra turbos and I'll use a T03 with a hat on my carb. The turbo will probably have to tilt down to fit under the alternator.
