Leaking transmission

My "wait and see" procrastination would have won again. Too bad you didn't change the front seal while you had it out since that is easy and the rubber lips get hard and crack, plus probably costs $5.

A 273 V-8 swap is not the "just" than many here relate. You also need a V-8 steering cross-link (some say most 1966 slants had them), and a 64-66 small-block oil pan to clear the linkage. Your shift cable would probably work with a 65 small-block tranny (1 yr only valve body, but your slant valve body should interchange). If your slant was factory AC, it probably has a throttle cable, which may reach a SB carburetor. At least your K-frame will work, if you have early-A SB engine brackets, and your driveshaft and rear-end would work.