got me a 225 slant 6 - a904 automatic - the trannny takes some 30% of engine power to operate the tranny - am i right in this saying? - thinking of the standard a833 - though with my bad knees i might be stuck with the a904 - there is the a500 - with adaptability to the slant6 - the a904 even with upgrades i am not too sure - the car is a driver with a bit of attitude

In what kind of vehicle, with how much wear and tear on it?
No, where did this number come from? The A904 takes vastly less than 30% of the engine power to operate. If the transmission were consuming 30% of the engine's power, it would get so hot it would melt into a heap of smoking slag.
Big, expensive, difficult swap. Take you a long time to break even on it.
What makes you think it takes less power to operate than an A904 which is identical except for the lack of the 4th gear?
Kinda-sorta. It can be done, but you're looking at floor pan mods, crossmember mods, custom driveshaft, custom controller...this is another big, expensive, involved swap.
Not too sure about what, exactly? The A904 is an excellent, very efficient transmission.

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Too many people who were born on third base actually believe they've hit a triple.

have a 75 2 door dart swinger - doing a restoration - what about the upgrades for the a904 - i am sure that perhaps adding or better yet improving the tranny would be the best way to go -

What improvements are you trying for...? What do you want it to do that it doesn't presently do, or not-do that it presently does, or do differently?

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Too many people who were born on third base actually believe they've hit a triple.

For a daily driven 75 Dart the only modifications I recommend being made during a full rebuild are (1) make sure the 904 has part throttle kick down and (2) install a shift kit. Otherwise, leave it alone. Like Dan said, the 904 is a VERY well designed and durable transmission.