Building a 273

With the small engine, and the full-frame Plymouth, to get moving briskly, you are gonna need one, two, or both things, namely; a higher stall and torque multiplication.
As for the 904, First gear is just 2.45, and Second is way out there at 1.45, thus the 1-2 split is 1.45/2.45= .592; ......... so the rpm drop at he shift is fairly severe.
With street gears, it is hardly likely that the tires will spin and if they do, it won't be for long; therefore, as already said, acceleration will be sluggish. To help with that, you will need a higher stall TC to find some power .
You might think the 4bbl will help. But think about it. At a stall rpm of 2000 (typical) the primaries are still keeping up, so even if the secondaries begin to open, the engine will not make any more power until the primaries are maxed out .......... which may not happen until around 2800/3000. Now think about this .... 2800 with say 3.23s is around 26mph with 27" tires. And when the 4bbl opens, it does Not immediately add power; that takes rpm. And furthermore it starts out by adding just 1 hp over what the primaries make by themselves, at whatever rpm they max out.
With a small cam, the engine may not make peak power until 4800/5000. In First gear, with no tirespin,that will be ~45mph (still with 3.23s), and at ~76mph in Second.
The point is this; with a small engine, you HAVE to consider your gearing, if you want at least a modest amount of performance.
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If I was doing what you are doing, I would seriously consider an A518, with ratios of 2.45-1.45-1.00-.69od and much more rear gear ....... like 4.30s. This will get you 33% more starter gear, and bring all your shifts down to a lower roadspeed, yet 65=2400 in loc-up, versus 65= about 2700 with the 3.23s and no lock-up @3% slip. With 4.30s, 5000 =34/58/84 in the first three gears; much better. The starter gear is 4.30 x 2.45=10.54 by the math, but the TC may, at zero mph, increase this to around 18/1, until the car starts to move. So in this case, you may not need any extra stall. Altho, with a loc-up, I would bump the stall up anyway, to say 2800/2600..
But in all honesty, if your 273 is wearing a non-273 top end, and has been bored; then the only thing about it that can be said to belong exclusively to a 273, is the block and crank............... so; As for me, I would just swap in a bigger 4bbl engine, leave it stockish, and still install the A518. I mean a 100% stock 8/1 318 with just a 4bbl and a free-flowing exhaust, might be in the neighborhood of 210hp; but with the 4.30s, it will seem way more powerful. Say about 30% more powerful than with 3.23s. lessee. 1.3 x 210=273hp is what that would feel like.
Jus saying. And that doesn't even consider the additional bottom-end torque ...........