QUOTE "You should be able to pin the 1-2 shift valve down(shifted position) and this will lockout 1st."
Pinning the 1-2 shift valve will work....
However you will never be able to select manual low ("1").
The trans will bind up in manual 1 because both 2nd and 1st will be applied at the same time.
That could be a serious disaster if the gear shift ever got accidentally knocked into 1 at hiway speeds.
If you physically restrict the shift linkage so that it is impossible to select manual 1, then ok, block the 1-2 shift valve in the up shifted position.