You can get a 318 cheap and the 360 heads will bolt on and work without having to notch the bores... I've even run the larger 2.02" 340 valves on 318's without any problems or notching the bore...
You will still have a compression drop with the 360 heads on the 318, but as I stated before the increased flow will offset the compression drop....
The 318 will bolt in directly for the 273 and will even use the same engine mounts...
But if you upgrade to a 340 or 360, they take a different left side engine mount which are hard to find these days... Shumaker makes the mounts for them, but they were sold off to a different company and I'm not sure if they have started making the 340/360 mount that you need...
Here's a link for the mounts, make sure to change the engine size box for the 340/360 mount as the 273/318 mount is the default... I tried to change it for you in the link, but make sure it's set for the proper engine size... Maybe call first before ordering them to make sure that they are available...
[URL][URL]https://www.engine-swaps....acement-or-upgrade-kit?variant=40745278177439[/URL][/URL]
Edit: I tested the link and it goes to the proper 340/360 mount size...