Slant 6 and 318 trans options
three on the tree, has traditionally always meant a 3-speed manual trans, column-shifted.
IIRC the 70 Duster was the First year that the A230 full-synchro was available, and only on a 340. But it will fit on any SBM. And it can be installed into your Duster with relative ease, no matter what previous trans was in it. The tough part is installing the correct clutch pedals.
I wouldn't try to shift that A230 on the column tho; it kindof defeats the whole purpose of installing it, cuz column-shifters are notoriously slow and problematic when pushed hard.
As for the A230, it comes in two flavors; Which are;
2.55-1.49-100 close-ratio and
3.08-1.70-1.00 wide-ratio
You cannot tell them apart by looking. You shift them into first and count the number of turns that the input gear makes with one turn of the output shaft.
Either trans will bolt onto the 318 bellhouse; and the bonus is, that the A230 is the same length as any trans that ever came in an A-Body except the A727. And it will work with the 1970 speed-O cable.
So if the car currently has a clutch pedal, the A230 is what you want.
If your car will be a cruiser, then the 3.08 wide ratio will allow a 20% reduction in rear gear for same take off and so the hiway rpm will be 80% of whatever you changed from. But the downside is that if you install a 2.76, she will be pretty gutless in third gear.
If your car will be more geared for city and performance, then you would want the 2.55 close-ratio, with a big-number rear gear.
So as a cruiser, with 2.76s already installed, the starter gear is reasonable at
2.76 x 3.08=8.50 And the rpm on the hiway with 26.7 inch tires will be around 65=2250
But if you installed the close-ratio box your starter would be a pitiful 2.76 x 2.55=7.04
As a streeter, using the 2.55 box and 3.91s, your starter gear is
3.91 x2.55=9.97 a very nice number. But your 65=3200
Second gear will come around 40 mph@5000rpm, and go to 4500=61mph or 68@5000 Very nice numbers for a stock long-block 318.
Good luck.
