He also made me eat my first 2 choices of motors with his 408 package. And the motor has no problem living.
408 or 416, there is a LOT to be said for them. Pretty good reliability and a lot of power and propulsion power to be had from them.
273 isn't exactly a step backwards if you don't have a roller cam motor. It might not be worth giving up the ability to run a roller though. It's smaller. It has less potential to be built. It would cost more to stroke or do a lot more work to. If you aren't and won't ever be building it extremely that doesn't matter. There are decent pistons for doing a stock-stroke build on it.
What would the price of a 340 be versus a 360? '72/3 motors are good to use. If you can score one stock in running condition for the cost of a 360, pull that trigger quick. If it's a '72 even better.
The 340 wouldn't have a lot of payoff over a '70-4 360 (a few other years before '78 may have had the higher compression as well, that's the main advantage) or Little Red Express motor and it would have different advantages than a TBI 360. It would probably have more power than a TBI motor. It could be easier to make power than a TBI motor. But it wouldn't be a roller block just as those high performance 360s wouldn't be.
However, a 340 with a tiny cam in it will be a lot like a 318 with a tiny cam in it with 360 heads. There's a good reason the factory put the 340 cam into 340s, and it wasn't just because it made it sound cool. They also put it in several of the HP 360s for the same good reason. A 340 with semi-steep gears and moderately sized tires will want and need to go over 3600RPM at times. The stock intake and cam were there so it could do that and more. Get conservative and indecisive and it will be just that.
Magnum era engines aren't a bad idea if the price is close. Converted to carb they do make power. You don't have to buy a specific intake, you can go buy a $10-100 used intake and redrill the heads or get LA pattern heads. (I'd do the latter now) Magnum heads are basically closed chamber 318 heads that actually flow well with revised rocker geometry.
Motorhome engines can have awesome parts on them, but can also have taken an awesome amount of abuse. A lot of these kept running forged cranks and could have room for those above mentioned overbores. But those overbores require sonic checking which requires spending more money than you might want to on a 318.