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The horsepower isn't the real issue. 273s love a little gear and a manual transmission. Use something a little hotter than a stock 340 cam, but not as hot as the D/Dart cam.

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Something around the MP .455 lift cam MAY be the ticket. The D-Dart cam IS pretty hot for 273 (even the hydro-cam .509" lift cam is too). IMO, the 68' 340 manual trans cam could be good here too, just degree it!! Food for !!

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The 4bbl 273 made 230HP from the factory so you are only asking 20 more HP, not aiming very high in my opinion. A set of headers and modern cam and you will easily surpass that with out pulling the heads.

If you are building a 273 from scratch get flat top pistons made or buy the cheapie ones and deck the block, get some 273 specific gaskets from summit so you can get .030 quench (very small bores and tall pistons can run even tighter but that should be safe and effective). Run a good quality small tube header, SCAT I beam rods, back cut the stock valves(even 360 size valves are gonna be pretty shrouded), do a little basic port clean up and an XE265HL cam on a 110 LSA. You would easily make 300 hp 300#TQ

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Nice tips on this build Dave!! Quite effective too, especially with the quench issue.
The cam issue is questionable though, there are so
MANY (grinds) ways to exceed this goal as well as being efficient by todays' standards.

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Back in '66, Mopar had a "D-Dart" version of the 273. It was grossly underated at 275 hp.
If it was that easy to get 275 hp 45 years ago, you can easily break 300+ today with just an 850 cfm 4 bbl. even if you screw up.

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True, for a setup like the D-Dart which was a DRAG
CAR (NHRA class racing). However, this is a street-type vehicle.
You can get 300hp without resorting to a 850 cfm
carb (save for a TQ). A 600 to 650 cfm carb will
do the job with the right cam/intake combo.

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The advantage in the cam I mentioned in a small engine vs the ones you mentioned are huge. The small engine will make very little TQ and the mopar cams will not help at all, a short duration high lift cam will help a bunch over the old mopar stuff with long duration short lift. It keeps you from loseing what little TQ they have but still allows them to breath at higher RPM where the small motor has to go to make any respectable HP.

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The MP cams were just an example of the parameters to work around,
NOT for an exact fit. They were mentioned in the above threads. Sure Comp Cams are very
reputable cams, BUT I left the OPTION of freedom of choice to the poster, since there are a VAST majority to choose from. That's all. Who knows, maybe he might go with a custom grind. His decision, anything's possible!!

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The 302 head has a much smaller port, in order for them to flow the same then the 302 has to have more velocity, very simple physics. The 315-920 heads have bigger ports and flow more than the 302 but they are slightly lower velocity.

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Dave, I think the guys are getting confused with the velocity vs. volume issue in the head issue. The 315-920 pieces are better at high rpm power ( more volume=more airflow capability), while the 302 has better low-midrange torque capabilities(more velocity=higher rate of air movement, quicker cylinder filling at low and intermediate engine speeds). I believe the 302's get the nod for this application.