Nope, sadly your '66 C4 won't directly bolt up to the '70 250 engine. The 250 uses the same larger bellhousing pattern as the V8s (289/302), while your original '66 200 had the small six-cylinder pattern. The torque converter and flexplate also won't match up.
Easiest route – just use the original C4 that came with the 250, it's already a match. Otherwise, you're looking at swapping bellhousing, converter, flexplate, and starter... a bit of a pain. Good on you for checking first before diving in!
Easiest route – just use the original C4 that came with the 250, it's already a match. Otherwise, you're looking at swapping bellhousing, converter, flexplate, and starter... a bit of a pain. Good on you for checking first before diving in!