Horsepower and drivability would need a supercharger. (I can't believe I said that.) Small cube engines need rpm to make a lot of horsepower. Valve size is the biggest drawback.
Depends how you do it, people build NA ford 4.6l and chev 4.8l to about 400 hp and a reasonably streetable but mainly do it with head flow with mild cams.
With a 273 and some sort of magnum or 360 head stock or ported and 215 to 225 cam should be fairly streetable and make around 300-350 hp.
As for the 273 I wanted to build in this thread drive ability was of secondary concern :)
