Looking for some Differential advice

La Mesa is around 550 ft above sealevel, so your slanty, if a 225, should have plenty of climbing torque with whatever gear is back there. I agree with those that are steering you towards a tune-up.
The things I would jump on are;
>making sure the throttle valve is opening all the way with the foot-feeder,
>a stall-analysis,
>a restricted exhaust,
> a compression test,
>a full timing curve analysis
not necessarily in that order, lol..

The stroker will not need 3.73s, so in all likelihood, you would be pulling them out in the future, for less gear, to hit the hiway with.
The 72Dart has a large rear overhang, that tends to make it an evil-handling beotch, even with upgraded suspension. The stroker will need the tallest and fattest tires you can stuff under the back. With factory tubs, and to fit period type wheels, this will require a spring relocation, and the rearend will need to be narrowed. Mini-tubs should be considered. She is however, a great 318car.

Bigger number rear gears help in three ways;
1) the car leaves the line easier and quicker
2) if you put second gear in the right place, you get better hiway passing power
3) at WOT, the engine will spend less time in each gear, so the zero to 60mph will decrease

3.73s will make a great city-car, but yes it is a bit much
With 3.73s and 26" tires; 32mph would be about 2250cruising in Second/4100WOT in First. The rpm of the peak power, of the stock 225, is pretty low. At 32mph=4100 in First gear, the engine is already fast approaching up-shift rpm. So when you nail it at 32mph, it may already be wheezing thru the single exhaust, and she will be up-shifting in a heartbeat. That's how I see it.
IMO, a higher stall TC would be better option, but the exhaust system better be up to the task. You can easily get 50% more off-the-line power with a lil more stall; perhaps even double. Back in about 1976, I installed a 2800TC behind a fresh 69spec 225 with an A904/2.76s, into my 69 Barracuda fb, and was very pleased with the result. Once you get the Rs up, 2.76s are 32=3100WOT@10% slip in First gear, and that gear will pull to 45mph@4400; so you got a nice window there.