Manual transmission swap
Why?
The 904 has a torque-convertor, which is a TOTQUE-MULTIPLIER;
that is like an infinitely adjustable device
that automatically shifts from as much as 2.0:1 to as low as 1.03
depending on the torque going thru it.
Typically you can expect a ratio of 1.8 at zero mph. That makes the 904 act as if it has nearly doubled the rear gear. Say you have 2.76s; at WOT, this could take off like it had 4.97s, then automatically, as the car accelerates, spool down toward 1.1 at top speed, shifting up or down by itself, as torque input varies, and speed changes. .
Swapping to a 4-speed manual; you won't get that, and with the same rear gear, the car will be lazy.
To get some of that TM back you will need to increase your rear gear. ............
which will increase your hi-way cruise-rpm.
If it was me, as in what do I think;
I'd just get a higher stall TC, and install a semi-auto shift kit in the 904.
But hey, if you just want a manual trans for the sake of having it, I understand that, cuz I have been into that same mindset for over 54 years.
For you, I recommend the AX-15, cuz it's a 5-speed suitable for a modest torque engine, and has an overdrive.
The AX-15 ratios that I have on file are
3.83-2.33-1.44-1.00-.79.. IIRC there are some other ratios.
This will work excellently with a rear gear of 3.23 (65=2200 with 25.5 tall tires.)
If I had a modest 318/273, or maybe a lo-torque 360 ; this is the trans that I would use. Adapters are available. Some fabrication is involved.
Diaphragm clutches have a modest leg-power requirement, I love mine.
If you just want to bolt it in, any fully-synchronized Mopar trans is a no fuss install. I and several others on FABO, have complete or nearly complete kits, and I think Dan Brewer as well.
Generally, it will not be cheap.
You might be interested in a the Commando/GVod I have in stock. It will make your 360 come alive.
There is nothing I would rather have.
But I ain't giving it away.