None , you have already decided on the fix. Let us know how it works out.

I wouldn't go any larger than the #40 drill, risks bleeding off too much pressure and making the rear band ineffective.

As to the throttle pressure, backing off the linkage might alter the shift points in auto but at least it would upshift in manual.

The 1-2 shift valve governor plug I mentioned was introduced in '70 but not on all transmissions...the improved version is on the left in the pic below.

Had to pull the valve body in order to get the rear piston out. Didn't have a #40 bit, so I used a 3/32, which is about .004 smaller. The Trans-Go spring and the little spacers were in place.

Then, as I got ready to put the valve body back in....surprise.... I notice that the valve body is stamped "68". What the ?!?!?
So I go to the stash and pull out one stamped "73'. Tomorrow, I'll swap the shift kit into the "73" valve body. That may deal with the 1-2 delay and I get part throttle kick down to boot.

John, I'll double check that shift valve while I am in there. Thank you so much for taking the time to share your hard earned knowledge.