Carb choice for a mutt motor.

273s and 318s used basically the same heads, so they're not the stumbling block with the stock cam (hydraulic/solid?).
A carb change may require a carb adapter, since the 273 manifold uses the early "small" AFB bolt pattern, some aftermarket (ie Holley) carbs don't come with both patterns. You may also run into some interference with the butterflies and the manifold openings, requiring either some grinder work to the manifold or a small spacer. I wouldn't go over a 600/625 (depending what flavor you prefer- Holley or Carter/Edelbrock) cfm carb. Stay with vacuum secondaries if a Holley. I would confirm satisfactory function of your current carb first, though; whatever it is.
More importantly at this point, get the setup and tune right. Sort out your timing/advance curve, that will pay more dividends than anything else right now. Something like a 10-15* initial with about 36* all in- experiment to see where it's happiest (remember to set your initial with the vacuum can disconnected and the port plugged). And make sure all your connections are correct- vac adv. to PORTED vacuum source (not manifold vacuum), coil polarity is correct,
Make sure your throttle cable is set so you're getting full throttle travel and opening all the way. If you have an aftermarket carb already, it may require a Mopar linkage adapter (if it doesn't already have one) in order to get correct travel.
What ignition is on it? Points? Chrysler electronic? Some type of aftermarket?
If points (single or dual), make sure the points are clean and gapped correctly and the rubbing block is lubed and not excessively worn- points ignitions are becoming somewhat of a lost art nowadays, so who knows how it may be set up.
Getting the tune and function right should be the first priority and may be all you need for now.