I agree the 273 is a good small engine, not what I would consider a powerhouse.
Can they built for moderate HP , yes, but then you have a grenade at the 400 HP level.
I have to disagree with the 360 under powering the 273.
What do you call powerhouse? 12 second quarters? 10 second quarters? 140 mph + in the top end? Ability to burn rubber through 1st and 2nd gear? I have had a High Performance 273 for 45 years. There is a guy down under running one in a vintage road race early Barracuda running about 470 hp. It dynoed at over 500 hp. You guys always assume drag race to the max. I did not get that from the OP. He does not need anything but a cam and valve spring change, a 4 barrel and intake for what he wants. I know more than most of you what drag racing is. Once you start down that road, if you are really serious, it never ends. The OP sounded like he wanted a nice relatively fast car that he could drive and enjoy. Don't worry, he is probably long gone. As for 273's and 360's I have driven both. There is no way a stock HP 360 would take my 273, which was not stock. I looked at buying a new car in the mid 70's with A HP 360. I ended up keeping the 64 Barracuda and 273 since it was faster. Instead I got the seats reupholstered in leather.