There are 16V heads available (B204 and B234 have those standard) but it's not nesecary. Volvo "531" and "405" (531 and 405 are three last numbers on the code casted on to the head, 360 with B19 has 106 and B200 has a 530 stock, which isn't as good as 531 and 405) 8V 531 head flows enough in turbocharged engine for up to 500hp when head work is done properly, so most 16V conversions for 8V engines are done in purely because of the "wow-factor."
If you are putting a 16V head from B234 or B204 in B23 or B230, belt tensioners and belts need to be tinkered, many people have used suitable Opel (Vauxhall) and VW parts. Also there are different versions of the 8v heads, ones with large coolant passages and ones with small. 531 allmost always has small ones, and that's much better for turbo use. (With large ones head can crack more easily, as the material thickness isn't as good)B23 and B230 have same outer diameter as the B19 and B200 series 2l Volvo engines in 300-series have, bore/stroke are just different. (There are so called "Penta" marine cranks available also which gives a 2.5l displacement with 2.3l block) And they also look the same, so it's kinda bolt on job, oil sump, oil pump, engine mounts and flywheel (If you want to use the stock engine managment coming with the B23/B230) are needed from 300-series engine. New stock 360 clutch will hold up to from 180 to 250hp depending on usage, but there are sintered stock size (210mm) paddle discs available and Sachs pressure plate from bigger turbo Volvos will fit in also. Also some VW 228mm discs have the same spline size as the 360 disc does, so those are also good and can be used with 228mm Sachs racing pressure plate for BMW. (I don't remember part codes but I can dig those up if needed..
)What comes to B230, I would stay away from engines from 86 to 89/90. As these engines have much weaker con rods which will hold up to 250hp in turbo engine in street use. 89/90 B230 and all B230's after that have much stronger con rods which will handle +400hp. (89/90 because there have been stronger con rods in 89 engines and weaker in 90 so both can be found from engines from those years, after -90 should all have stronger ones)
B23 then again has always very strong, so called "M-rods" which are a bit stronger than the newer model B230 rods. (well pre -80 engines have weaker rods, but engines as old as those aren't around much..
) Also aftermarket H-profile racing rods are available for these engines.B23 series engines are pretty much same design as the B230, they look the same and outer diameters are the same. B19,B21 and B23 series engines were used from -75 to -85, -86 was the model year when B200 and B230 series engines were introduced. B234 is the same basic design also but has some differencies to 8V blocks. (For example it has external oil pump etc...) There are italian market versions of the 16V engine, which are 2 litre and have turbo. Those are B204FT and B204GT, FT produces around 185hp and GT produces around 200hp stock. 2.3l 16V never came with turbo as stock.
I hope that helps, if you have some questions or if I was unclear about something, I can try to explain better.
Of course you can always turbocharge the B19/B200 engine in 300-series also, because all the bits and bobs from the bigger Volvo will fit in, but there's not much point as with same amount of money and time you can fit the whole lump from the bigger Volvo.
I've got a B200E with Garret T3 in my 360GLT, but my car was originally built in somewhere early 90's so prices for complete engines were quite higher back then.. Here are some links for shops in Sweden which sell tuning/racing parts for Volvo redblock engines:
http://www.stenparnermotor.se/
http://www.kgtrimning.com/
http://www.samsteffansson.se/
I wish the Volvo would've copied the strength of the 928 drivetrain also. 
