For a built motor, nothing is wrong a B20 block that sleeves can't fix. As Wanye said, "there are a few people on this board who were are are runnin b20s as DD and they are running great" It's all in the way the motor is built...and how you treat it afterwards.
As far as running a Turbo'd B20 with a B16's 77.4mm crank...doing so with "off the shelf" pistons...you have to use something close to a 142.8mm rod. Now with that...a 142.8mm rod and a 77.4mm crank will give you a 1.84:1 rod ratio which will help with revs but will def kill your off the line/bottom end torque.
Not to say that it won't work...cause it will. But for every cause, there is an effect.
Also here are some displacement numbers to slow you what will get if you start swapping cranks around.

a B16A 77.4 mm crank with a 84.0 mm bore = 1707.063376cc
a B17A 81.4 mm crank with a 84.0 mm bore = 1804.400214cc
a B18C 87.2 mm crank with a 84.0 mm bore = 1932.969181cc
a B18/20 89.0 mm crank with a 84.0 mm bore = 1973.068310cc

If you truly intend to run a turbo motor then don't worry so much with porting the head. Head porting is not as important to Force Induction motors as it is to N/A motors. Save the money and focus on things like, oiling, cooling, and a stronger bottom end. On a Turbo motor...that's what is going to keep your motor alive!

Enjoy!