I've lived your current quandary back in the day, and currently, I own a 65 Skylark with 300 4bbl and 4 speed, 65 GS with 401 4 speed and 64 Skylark with a 455 & TH400. The simple question is, what do you want to do with the car, or what kind of driving experience do you desire? The 4bbl really wakes up the 300. Add some rear gears (3:55 maybe or 3:73) for real fun. A 4 speed makes it way more fun and parts are easy to find since it's a BOP motor, not a nailhead. For just a pleasant cruiser it's a great combo, but it's a far, far cry from a big block. If you built the 300 as radical as possible, a mild 455 would still beat it. I've done it. Driving my 401 is about as pleasant an experience as I can describe, but it is not a 455 either. Nailheads can be made faster than stock, but the design is from 1952. If you're looking for a street/strip machine or a car that roasts the tires every time you stomp it, you should be looking at the 455. It's also a BOP motor and much easier to get parts for than a nailhead. Nailheads are nailheads, and many here love the design, but they require special transmissions, bellhousings, motor mounts, etc. And go-fast parts and options are limited and expensive. A well-tuned, mostly stock 455 in your car with decent gears and traction will run low 13's over 100 mph all day long. A 300 will be pressed to be in the 14's. The torque under your right foot sums it up. Have fun and good luck!
Pat