Here's a picture of the "cleaver" style fin.
The photo is from the 2002 Website archive.
The fin in the photo is a 40 cm, but these fins have beens supplied in 33 cm; 40 cm;
41 cm; and 43 cm.
All of the cleaver fins are basically the same shape (minor changes in the radius at the
juncture of the leading edge and the tip, but basically the same).
If you look at the GO board section of the 2002 website archive:
you will see the 33 cm Drake Ames version of the cleaver fin, and also the GO side fins.
Not sure if your GO 155 has the side fin inserts, but earlier model GO boards all had the provision to attach the side fins for sub-planing early beginner sailing.
As far as getting out the inlet on your lake, learn to tip your board upwind and carry the rig a little forward of vertical and you should be able to get upwind fairly easily in sub planing mode.
You Chinook bases should fit, but you may need the brass T-Nut to extend the length of the threads to accomodate the thicker EVA deck material.
Hope this helps,