Thinking about converting from 4speed to 904 or slant six 727.

If you have a big slanty, it will do well with an A998 close-ratio LU and decent rear gears. The ratios are the same as the Regular A904, but it should have a decent stall, around 2400.
Because of those features; compared to the A833od box; the auto acts like it has 4.5 gears
Lets say you currently have 3.73s.
Your roadgears are 11.52-6.23-3.73-2.72 in the which, the cruiser gear is 65=2200
Take off ratio is 11.52, and 35mph is about 2700; all pretty good numbers.

Now, let's say you really like that 6.23 Second gear.
The convertor, is a TORQUE convertor, that inside it, has the potential to nearly double your engine torque at Zero mph. Let's assign a value of 1.8
So the rest of the roadgears then are;
16.45 at zero mph, dropping to ~12.8 at shift rpm.
Coming into Second, you might see a ratio of 7.00, and into direct @ 4.47, and into LU at 3.73
Ok so lets line these up; These are all at WOT.
16.45>12.80-7.00-4.47-3.73. I count 4.5 gears; compared to
---------11.52-6.23-3.73-2.72
You see what I see?
Well firstly it's gonna cruise at 65=3020 in LU, and secondly
Second gear is ~7.00, compared to 6.23 with the od box, so we could give up 12% without noticing a thing., and
Lets say the average thru first gear is 14.62, compared to 11.52 with the odbox, so we could give up 27% of that gear and take off would feel the same.
Now getting down to 2.72 from 3.73 is not so easy, as that is a difference of 22%. You could give it up in First, but Second will definitely hurt.

Ok then, Say we give up the just 12% of the 7.00 second gear and see how that looks. This will take a rear gear of about 3.33, and lets round down to the available 3.23s.
The OD numbers are still;
----------11.52-6.23-3.73-2.72 . compared to the A998LU, at ;
14.24>11.08-6.09-3.88-3.23, and 65=2615 in LU. IMO, this is an acceptable trade, for the slanty; and the bonus is that you have two passing gears, maybe three.
So say you want to pass a guy going 55mph. and you're gonna floor it;
If the trans only comes out of LU, the Rs will rise to 55= 2430@10% convertor slip/ 2650 at 20%.
If it goes to Second NOT-LU, the rpms will rise to 3850@20% slip but, if you are able to keep the LU engaged
then, 55=~3210

All-in all, there are plusses and minuses.

If you do not currently have at least 3.73s with the A833od, my question is why not? , lol.
Happy HotRodding