(2 1965 Barracudas, /6 vs 273.) I had the same path you are faced with. I went with the /6, which is still an ongoing project. The engine is ready to be installed and run. Head and block work, cam (Hughes), intake (Offy 4 barrel), headers (Clifford), N2O, etc. If I were to start over...I would finish the car first, up to the point of installing the engine and making it a "roller." Right now, my project car is still on the rotisserie, not quite ready to be on the ground. In the meantime, my ready to run "hot rod /6," has been on the stand over a year. It seems (IMHO) everyone does a small block or something else. I want mine to be kind of unique, thus the /6 (which is a matching #) for this car. The 273 is the same for the other car, matching #. if you choose, swapping out a /6 for a 273 it's not a real big deal. However, I elected to go from a 904 A/T to a 833 4S which has been giving me some headaches. My 273 car had a three speed in it and the project car was an A/T. Some of the linkage and stuff works, some of it does not.