Roger is producing new racing gears for the A604.
Check out his site.

http://www.profitsonthenet.net/gears.htm

also if you reverse the compensating spring in the tranny, it will shift harder and faster (and also won't wear you gears out as fast)

yeah, but the a604 will fall apart with minimal abuse, so why bother?

  In my own  words I would put this forward, Its almost crimminal  what detroit puts
in these cars and what you pay for them,  These cars are made of crap and all
the transmissions are the same and their is simply one kind of automatic, those that
break and those that will break,,, The reason you install gears it creates a performance
advantage that is hard to bridge with anything else. If your avenger has the gears
and the other identical avenger dont,, you win  It allows you to get up to power
range faster, it will firm up shifts and makes life for the planatary gears much
much easier... The 604 I put together with the quick change gears gets a
second gear chirp  and  its gets the hell abused out of it,, If  a person will
alllow myself to spend the bucks on the parts and my own personal tricks you can
get a 604 to hold up just fine,, If your reuilds are not holding up I suggest you get
in contact with me  and Ill prove that a 604 can and will hold up. Ill be waiting
for a call....573 248 2149