904 transmission input shaft
which I really don't want to get into
Which is too bad, cuz that's a dynomite combo.
The .69overdrive let's you run up to ~4.30s on the street, altho 3.91s are my favorite. But your 400 hp 318 won't like 3.91s much either, unless you run a hi-stall. The more gear that you run, the less stall you can get away with. and a LU convertor makes cruising, say with 4.10s and 27" tires to be;
65= about 2300, and 75=2650 your 400 hp 318 won't get very good fuel mileage at 2300, but it will be about as good as gets, cuz at least the cruise-timing can be made to be close to correct.
But if you never or rarely hit the hiway, then I guess this is a moot point.
FYI;
The A500 at WOT, functions like a 5 speed with the LU adding another half a gear. The extra gear comes from the convertor having a torque multiplier function inside of it that has a ratio range of about 1.1 to 1.8, or even higher. At Zero mph, the ratio will be around 1.8. As the car picks up speed, the ratio will diminish, eventually ending up nearer the 1.0 ratio. The multiplier also varies with torque input. Of course the convertor in the A904 has the same feature as do all convertors.
Because this combo can put down so much power so early, on the street, you really don't need anywhere near 400hp, to have major fun with, and that has the side-benefit of the 318 being waaay cheaper to build, and it will last a lot longer.
Furthermore;
The difference is that to run 65=2300 with a non LU A904 convertor,the rear gear will need to be about 2.76s for 65= 2300,slipping at 3%, and your HO318 is absolutely gonna hate that gear at take-off, no matter how much stall she gets.
But I understand, cutting and fabricating ain't for everyone.
But with a 400hp 318, I'd be willing to bet you a dollar, that before very much time goes by, yur gonna thing about that A500 again.......lol.