So, I'll tell you a secret
3.55s are a great middle of the road gear. And the od trans can be made to work quite well on a low-rpm/hi torque engine.
If you insist on keeping the teener, then you're gonna have to boost it.
Or you can heave that od box, go back to the A230, and redefine why you want 350 hp.
To get that 350, means sacrificing low-speed driveability. You can't have both with a 318 and 3 gears. Not even with 4 close ratio gears. You will need 5 gears. Think Mustang;302 engine and 5 gears. There is no way around this.
But if you broaden your horizon to a 360, then this all works out so perfect.
360hp is easy for a 360 with a small cam and good heads. Then you can use that A230 and the 3.55s or even 3.23s.
Or you can use the od and 3.91s. These will get you a second gear of 1.67 x 3.91 =6.53 which is very nice for blasting off at 32mph=2650 with a mild 360 cuber. Your hiway gear will be 3.91 x .73=2.85 so 65=2350
and your starter gear is a tire-frying 3.91 x 3.09=12.08 which will take you to 35mph easy enough, at about 5500.It's way more starter gear than you need,but that's the way it is with that od-box.
Stop worrying about blowing up the 8.75, properly set up and on the street, it will never happen. I've been pumping over 400hp into mine with 295s back there for probably 100,000 miles or more and well over 10 years; it's a non-issue.
But if you just want to have a lil fun with a 318, then forget the 350hp target for a bit.
On the street, you spend like 85% of the time at under 3500 rpm. A 350 hp engine will not make 350 until maybe 5600rpm or more, which is way down the road. No you need power from idle to 3500 where 85% of your driving is gonna be.Am I wrong? I'd be willing to bet that 95% of the time will be spent below 4000 rpm.
Ok so how fast will you be going at 3500 rpm with the A230 and 3.55s? Well the math says about 26mph. With the right small cam, you will be very near to your torque peak and ready to blast off. At 5000/36mph will be the 1-2 shift. The Rs will fall to 2900 and and if you had a 350 hp teener, you'd be sunk. But the small-cam teener gets right to work pulling that up, and by 63 mph is again at 5000rpm.
So while the 350hp engine might get you to 60 faster than the 275 one, it probably won't be with the A230 and 3.55s, and it won't have near the driveability below 3000rpm. And it will suck gas supremely on the hiway.