273 to Slant Six Conversion?

Sweet, so I found a '71 s6 and tranny.

That'll be fine. Should even be able to keep the existing driveshaft.

I found early A s6 engine mounts on eBay for $45 bucks - does that seem too cheap?

Seems too costly; all you need is the brackets that bolt to the engine block. The insulators, you should get via the link I gave above (polyurethane positive-lock items, better than the plain rubber stockers and WAY better than the compressed cow poo coming in from India or compressed unwanted baby girls from China).