Sometimes you can work through the clutch fork opening to change the counterweights. If you don't have room to do this then you will have to pull the trans each time. If you cut a "window" in the bell for access you will lose the sfi certification and will compromise safety in the event of a clutch/flywheel failure. There should be 6 adjusters in that plate to set base pressure. You can usually get to those by removing one of the trans to bell housing bolts and use a long allen wrench. You will have to index the your balancer with the trans out to line the adjusters up with the bolt hole so that you can make this adjustment with the trans in. You should check out the forum at
http://www.umtrnorth.com/ . You'll have to sign up to view the forums. There's a lot of good stick car info there.