He may have one on the shelf ready to go. He does carry some inventory now and again. I have a complete grille and headlight buckets for a 70 300 but not a motor unfortunately. Have you posted any pictures of your 300 here?
I sourced my motor from a '78 New Yorker. The '69 motor takes a different torsion rod than the '70+, and I'm not certain, but I think the '69 rotates in the opposite direction of the '70 and up. There is a sticker on the top of the hood latch deck area that shows rotation of the motor.
While the motors seem to be interchangeable, different applications had differently shaped switching cams sitting on top of the gear, allowing for different length of travel. These cams can be identified by their color. In a 1972 MTSC booklet the following are listed:
Red -- Plymouth and Dodge Charger
Black or Green -- Dodge Monaco
White -- Imperial
Not sure, but I think a 1970 Chrysler also uses a white cam.