Don't know about the Mk IIB but with my Mk III the cap job was quite easy. The solder connections were easily accessible on all caps.
You will need 2 220uf/350V caps (4 for a simul class model) and 3 30uf (nowadays 33uf, which is within 10% tolerance and OK) at 500V or more.
Make sure that there is no voltage left in the amp before you replace the caps. After playing, shut the amp off with the power switch but leave the standby in the on position. This will drain the caps through the 330k resistors in parallel to the 220uf caps. I always check with a multimeter.
Good luck,
Stephan