What size small block?

You have what's called an early A body... Runs from 64-66 if memory serves correct. The small blocks come in several basic forms but what will come close to bolt in are the LA and Magnum V8s. Those are 273, 318, 340, 360, 5.2L(magnum type 318), and 5.9L(mgnum type 360). However, there are caveats... In your case the biggest would be the trnasmission, torque convertor, or bellhousing. The hub that seats in the crank of your 273 will not sit properly in a later engine without some sort of adaptor or changing of parts. The guys on the "Early A" section can be a better resource for you.