You are right there is no adjustment built in to the frames for the 54T and 12T gears or the upper main shaft bearing blocks. However in my case there was enough clearance on the round holes to adjust the very tight mesh i had between the 54T and 12T. I was able to do this by loosening the main shaft bearing block and TT assembly screws and placing a paper shim between the spur gears whilst tightening them. I then re-set my motor mesh.
However, you also noted in your previous post that the noise or vibration levels didn't change after your paper shim adjustment. I submit that this is because when you tightened up everything, despite the paper shim, the assemblies went back to where they originally were in relation to each other. The paper sometimes just gets crushed.
I know that the paper method works for some folks when you have real adjustment available. However, I don't think that method is the most accurate way to accomplish proper gear lash. You must always find the 'high spot' on the gear. Then, set lash there. Move the motor up until there is obviously no slop, firm but not jammed mesh. Next, rotate the main gear 45 degrees at a time to see if the mesh feels tighter or looser. You may have to back off the motor just a little. You will find the low spot as soon as you can feel a slight tick when gently rocking the main gear back and forth after moving the motor back a little at a time. the tick is because the teeth of the main gear and pinion have been moved apart just enough to not be tightly jammed together.
Usually, the HIGH spot is 180 degrees from the low spot. Move to the high spot and move the motor back just enough to feel the slightest gear tick when rotating the main gear back and forth. Now, rotate the main gear and see if it is tighter or looser at 45/90/180 degrees from your first adjustment. If tighter, you didn't make your adjustment at the 'high point' of the gear. It takes a bit of trial and error but if you do this correctly, the high side of the gear will be just a hair's width from being tight and the low side will have a bit more gear 'tick' or clearance when you rock the main gear back and forth.
Obviously, this method can't be used to adjust the mesh between the 54T and 12T. The only way this gear set mesh can be adjusted is by elongating some of the frame mounting holes. I think you'd have to elongate both the transmission mount holes and the forward boom clamp holes since the transmission is butted up against the forward boom clamp. Or, or you could face off the front of the forward boom clamp a few thousands of an inch and just elongate the transmission holes.
