hi, i ran a 356 motor in my old rat bug a couple of years ago.
first i had to cut away some material around the rear bumper mounts to clear the cylinder heads - i also removed the side trays inside the engine bay... those motors are bigger than they look when you start offering them up! for a clutch i used a bugpack h/d cover plate as mine had a 180mm flywheel (it was an older,50s, 6-volt 356). those bugpack clutches are rated at 1300lb, and it was cheap, too!
you need to trim the edges of the cylinder head shrouds a little, but that should be enough to get the thing bolted in place.
mine was running ndix carbs and the stock linkage, which i hooked up by adapting the lever rod end from a 356. the only downside was that the linkage ratio didn't match the beetle throttle pedal (ie it didn't have enough travel to reach full-throttle. an easier option would have been to fit webers and an aftermarket bug-style linkage that hooked up at the rear of the engine! but, more important at the time was that the motor looked 'stock' and as though it had been fitted back in the day. the longest job though was fabricating an exhaust. in short, i gave up trying to mod a 356 'zorst and decided to use a bug header with the square-port ends from the 356 zorst grafted in place.
that engine was only a 75hp unit but it was really nice to drive. very smooth and revvy - more refined than a tuned bug motor.
go for it, it's not as easy as some will tell you, but if you don't mind cutting your car a little, it's a cool conversion and a great talking point in the pub!