Transmissions swap

Mason; I think you will be unhappy with the results. You will loose some take-off power, unless you slip the clutch to get the Rs up.
There are two A903s with ratios as follows;
.......2.95-1.86-1.00 and
.......3.22-1.84-1.00, compared to your A904
.......2.45-1.45-1.00 But at WOT,that changes to about 4.41/3.18-1.74-1.10 after including the Torque-Multiplication going on inside the TC.

With the manuals, direct and second are pretty far apart at ~54% so yur gonna have to rev second out if you need any pulling power in third, cuz the Rs are gonna drop nearly in half..

If you just wanna have fun shifting, that will depend on what rear gear is in it. With 2.76s, and 24.5 tires,2nd is;
2000 at 29mph /3000 is ~44/ 4000 is 59.
So most of the time, you will be in second gear. Not a whole lotta shifting going on.
There is no synchro on first in those boxes, so backshifting into first is gonna be a huge downer, until you learn to double-clutch and rpm/speed match. You can never do it with any kind of accuracy or consistency.

So IMO, I wouldn't do it; the slanty needs a Commando box;
3.09-1.92-1.40-1.00 and 3.73s will get you 60 at 4300 in third so, that's two shifts at WOT, 3 if granny shifting;
But in second 2000=20/ 3000=30/4000=40; so same old/same old.
However, with the tighter gear-splits, you can now granny shift. The splits are about 72%, except the 1-2 is 62%
So if you rev to 3600, the drops will be to about 2600, so then 2600 will be 53 in 4th, which being all three shifts. See; now yur having fun. Allbeit short shifting, so not going anywhere fast.