Advice on small block (and transmission) shopping?

Hey, guys!
some quick background-

I have bounced on and off this forum for a few years now...not being sure what I wanted to do with my '73 Dart sport. The car was my pride and joy when I was 18 (back in the 90's) and I was never able to let it go, even after it was parked as I moved on to sports cars and Mustangs.

Well...as much as I love my 2011 Mustang GT, the Dart is still my car and selling it would probably kill me. :p I'm getting serious about getting it back on the road in time for the spring and possibly ditching the blue oval.

The body is in good shape, interior is still clean (minus a tear in the driver's seat)...but the 318 has had it.

So I'm looking for a fresh(er) small block. Currently the car has a tired 904 (slipping, probably needs a rebuild) and the 7.25" axle/open with 2.73 gears

So as I start the part search, I'm hoping for some advice on picking a donor motor....years, versions, etc. for a relatively tight budget.

I'd like to drop in a motor that will have a broader power band than the 318 (though I did love that motor), reliable...but still have room for performance mods down the road...so something with stout internals.

Is there an optimal version of the "Magnum" motor that is the strongest?

Secondly, would I be better off having the 904 rebuilt with stronger components or finding a 727?

Third, my rear axle assembly search has been frustrating...I will certainly replace this down the road (probably with a custom axle since A-Body 8.75" assemblies with the larger 4.5" bolt pattern are all but impossible to find around here)....but in the mean time, how much power will the little axle withstand, assuming I don't abuse it?

Thanks for any input, guys. I'm starting over from scratch!

By the way, I abandoned the idea of buying a junk yard 5.7L Hemi (can be readily found for ~$2k) since the consensus is that the extra work doesn't justify any performance edge (do you guys agree?)... which is unfortunate since even a stock 5.7 puts ourtat least 340hp and supposedly bolts up to a 904...

Mike