I really have a soft spot for the 78 formula W50 cars, as that was my first car. Mine too was a 305 but a 4 spd. If I wasn't worried too much about keeping it a #s matching car, then I would do a 383 SBC or something. But seeing that you would like to keep it #s matching I can respect that. As for the 305, there isn't too many options for boosting power. You could open the heads up a bit, change the cam, lifters. Put on a 4bbl intake, and I would do some headers. I am not too sure how much you can even bore a 305 block, but you could find out from a machine shop then go that route. But what ever you decide to do, make sure you keep all the original parts just in case you want to put everything back.