What would I gain from a 4bbl on a 273?

If 273 is stamped into the left front face of the block, it isnt a 318. One thing you need to ask yourself; does it need to "look original"? If not, (assuming you have the later bolt angle) you coud certainly do worse than a 1406 Eddy on a Performer manifold, and the factory single plane iron intake would be worse. This would certainly support a bit larger cam down the road, and not kill your low end. Heck, when the old 273 gets tired a 318 short block would still be well served with those same parts. Headers are something to think about I suppose, but for a mild street driven early Dart, I can think of other places to better spend the cash (BRAKES!!!).

Bottom line here, a small 4 barrel and dual exhaust on an otherwise stock 273 or 318 will certainly make an improvement, but don't expect "instant race car". You won't make it worse in any event.