[AT] El Diablo shift key

[DonThiel]

All:   I have one of the famous IH model 666s, lovingly referred to as 
El Diablo.    There is a key (approx 1/4 x 1/4 x 1 1/2) in the shift 
linkage that keeps falling out when I move the shift lever to the 4-5 
side of the range.  Then I get to dig around to find wherever it fell to 
and try to get it slid back in place.   This takes three hands and a 
flashlight.  Cuss words optional.   The key is supposed to stay in place 
by friction, but I am guessing the slot is worn.    Other than never 
shifting to 4-5, anyone got any good suggestions for me?   Do I have my 
trusty machine shop make me a larger key?   How much larger?    Cannot 
reach it with a calipers to measure the slot slop.

Thanks.

Don