I have a O-340 (now an IO) in my 235. Since I had a dynofocal mount with
the previous engine I had the case modified from conical to dynofocal by
Ajax in San Antonio -it cost about $700 with a complete inspection
included. The O-320A is a narrow deck engine like the 340 and I didn't have
much of a problem fitting it in. But I had to use a short 320 cowl (two
inches wider and taller) since I couldn't find a 235 cowl to replace my
damaged cowl (there are a number of 320 short cowls around collecting
dust). Fitting the 320 cowl was some what of problem it took a few extra
days vs a 235 cowl. Since I had to add about 8 pounds of glass and filler
to repair the tail area (it broke off) the added weight of the 340 kept the
CG just where it was with the original 235 engine. I'm still using a wood
prop, three bladed now. I don't think you need a constant speed except to
cut down the engine noise at cruise 2500 RPM vs 2750 RPM with the fixed
pitch prop.

Rusty