Looks like the 327 275 has been gifted with HP. (NHRA.com) The 1966 275 is now at 299 HP with OE heads & 304 HP w replacement heads. The 327 300 HP is at 285 w OE heads & 290 W Replacement heads. Might make the 300 a better choice, depending on the differences in the engine combos. If you know the shipping weight of the heavy car you want to use you can use Bill's GT calculator to figure out what classes it will fit. You have to know 3 things to figure the class w/GT calculator. 1. Shipping weight of the car you want to use. You can get the factory shipping weight from the NHRA classification guide.
http://www.nhra.com/tech_specs/classification/ 2. The factored HP of the engine. (get the same place) 3. Either Auto or Man. The program works. It even gives you choices of which classes you can move to by adding or subtracting weight & the HP required to run. Pretty nice tool.