For context, I am putting a mild 318 in my 74 Duster that has the 198/ 3 on the tree and wanted to go ahead and convert to 4 on the floor ....
If the 318 has a "mild" cam, but is still an 8/1 compression engine, then, I wouldn't recommend this box except to use until you get something else.
The reason being;
Firstly because, I expect the cylinder pressure to be quite low, which will make the engine's low-rpm torque quite low.
and Secondly; While first gear with say at least 3.23s will be reasonably ok, it will fall on it's face when you hit Second gear.
This combo will want at least 3.91s for Second gear to be fun. But then, First with 3.91s, is almost too deep. So it gets to be a ratio/rpm dance.
Consider this;
Say you end up with 3.23s in the back. So you take off normally, contemplating a shift at 2800 like I do, which will be about 22mph.
But then, on the shift, the Revs drop to ~1500. Now, your low-pressure 318 is asked to come out of that, and it's gunna take a lot of throttle, and still be a dog cuz it won't hit 2800 in Second, until over 40 mph, and at 2800@40, that low-compression 318, is still not pulling.
It won't hit 60 until about 4400.
So yur asking that "mild "318 to pull from 22mph to 62, a pretty tall order.
In other words, yur almost never gunna outshift first at 2800.
Try 3600@28mph , dropping to 1950 on the shift into Third. The mild, low-compression 318, is still gunna struggle when it hits Second.
I gotta tell ya, 3.23s suck with a low-compression 318, and that trans.
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On the off chance that you have an older 9/1 engine, now with a mild cam, and your pressure is up to 150psi, OK, now it's better but still no hotrod with 3.23s.
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I can ALMOST guarantee, that you are gunna, eventually, figure out that Second gear is where that box will do it's best work, and you will end up with 3.91s.
In Second, with 3.91s, 30mph will be 2450, and 60 will be 4900. So yur still stretching the Second gear out pretty far, and coming in pretty late, say from 4500 in First; that trans is a tight fit.
For performance, with a 318, don't even think about getting into Third until 60 mph at the earliest, cuz as soon as you shift, the Rs drop to ~60%, so from say 5000 in Second to 3000 in Third, and there you are again, struggling to build rpm, way off the "mild" cam's sweet spot.
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like I said, it's a low-rpm box. As long as you use it as it was intended, it works fine. But once you start winding your engine up, the very wide ratios rapidly pull your engine out of it's rpm band.
