Back up the bus a sec;
Unless your 225 is supercharged, your car does not need rear disc brakes.
Your 9inchers are more than adequate. With 4 same sized tires, the best you can do in the back, is to keep the back in the back; any more than that, and it will put you into a spin.
Even if you split your system like the later cars are, and install disc brakes all around, you will still have to reduce your rear brake power, so as not to have it come around. This is of major importance ; if the rear end locks up, you are milliseconds from spinning out.
With 4 same sized tires, your front brakes will be doing over 85% of the stopping. In a lightweight 64 such as yours,that car does not need a whole lotta brakes but, if you can lock the front brakes now, you need more tire, not more brakes; the first thing to do is to get rid of the factory pizza cutters.
As to gears;
Gears work to multiply your engine's torque to a higher level, just like levers and pulleys. But in doing that, the engine has to be shifted sooner, and you run out of gears to shift into, sooner . So your hiway rpm gets a lil busy real quick.
For you, a higher stall TC is IMO a better choice than gears just one size bigger.
Here's why.
Say you have a 1700TC now. And say your engine makes 30 hp there. 30hp at 1700rpm is 93 ftlbs.
If you change the rear gears from 3.23s to 3.55s. that is plus 9.9%, which is the same as multiplying your 93 ftlbs to 102.2, which is now, 33 hp.
But say your engine has 40hp at 2000 rpm, I mean I'm just guessing to make a point here.
At Zero mph, which would you rather have; plus 3hp with gears or plus 10 hp with a TC. Not a trick question.
Where the gears are gonna help is; as the engine revs up, the 9.9% bigger 3.55s will run to shift rpm quicker than 3.23s.. So your slanty will deliver it's horsepower to the road in a shorter distance, and that will make it quicker to the shift rpm. And the same will happen in each successive gear until you run outta gears.
But your hi-way rpm, at the same mph with either gear,will be the same 9.9% higher with the 3.55s.
Satisfaction;
By the time you add up the cost of a higher stall TC and a new rear end with higher gears, you'd be well on your way to a V8 swap, that won't need either.
Look, I'm not a slanty-hater. I'm just saying from a dollar to performance increase ratio. If you need to stick with a slanty, no problem, do whatever makes you happy. This is in no way meant to be derogatory, I get it. I have hopped up a few slantys in my 50 years of driving.
Getting back to the 10hp question above. To get the same 33% power increase at zero mph,namely from 30 to 40, with just gears, would require a rear gear increase of 33% so from 3.23s to 4.30s..... and your hiway cruise-rpm will likewise rise 33%. With 75" tires that would be from 60=2730(no slip) to 60=3630(no slip).
But with a TC change from 1700 to 2000, your cruise rpm is NOT affected.
Note; the numbers I used may not be what your engine has, I just used stuff off the top of my head. Never-the-less the math is right and the high-points are also the same.
As to overdrive,
there is no od that will fit in your car without cutting the floor boards to make room for it. But the thing you can do is install a deep-low 4 speed manual trans, or you can swap in a 5-speed manual.
Neither is cheap, and the 5-speed is not easy.
These will give you a lower first gear, and tighten up all the others... compared to your A904.
More about gears;
Your 904 ratios are;
2.45-1.45-1.00. But the TC makes these perform similar to
4.41 to 3.18-1.91-1.10 .......The Commando 4 speed is
.......... 3.09-1.92-1.40-1.0 ...... and the AX-15 is
...........3.83-2.33-1.44-1.00-.79od
Lets match some rear gears on them , and see what happens.
First the A904 with 3.23s, engine at WOT;
14.24 to 10.17-6.17-3.55, next the Commando with 3.55s
............ 10.97-6.82-4.97-3.55, next, the AX-15 with 3.23s
.............12.37-7.53-4.65-3.23-2.55, and the AX-15/ 3.55s
.............13.60-8.27-5.11-3.55-2.80
I lined second gears up for you, so you can see the progression in performance as the numbers get bigger. That 8.27 is 34% bigger than the A904 and that means 34% more road-torque, at the same roadspeeds. Second gear is usually where all the action is for a streeter.
That 2.80 gear will get you 65=2560 with 75" tires, 2260 with 85s, sweet!
The AX-15 will not bolt onto your slanty; you will have to make an adapter. Nor will it bolt to your crossmember or driveshaft. And you have to cut a hole in the floor for the shifter to come thru.
Now lemme just throw something at you;
Suppose your slanty puts out 100 ftlbs at 3000 rpm(this is 57 horsepower), (see note 2).
Suppose you were able to install that AX-15 with the 8.27 second road gear. Then, at 27.5 mph(80" tires), when you floor it, into the rear axles would go, 827 ftlbs. Remember that number; 827.
If you dropped a 318 Magnum in there, with the same 3.55s and 80s, that beast makes about 295 ftlbs at 3000, so to the road this is 2440... compared to .....827. This is so huge a difference, that you could regear that 318 with 2.76s, and get, in second gear at 27.5 mph, 1450 rpm, 1048 ftlbs,into the rear axles. That is almost 28% more ftlbs at ~half the rpm. And finally;with 2.76s she will cruise at 65=2525 with 75s, 2370 with 80s, and 2230 with 85s ..
The point of this story is to kindof show what gearing can/cannot do, and how torque works with gears to increase your performance
Note 1
The Magnum will NOT bolt up to your slanty A904. But it does to the AX-15.....
Note 2
I suppose your slanty could make more than 100 ftlbs at 3000, but the point is still the same. Whatever it makes, it will still be eclipsed by the Magnum.
