383 or 406 is definitely a good way to go(383 is probably easier, 400 blocks arent that easy to find), either will be bolt in motors(same motor mounts and accesories as your 305)
Just focus you build on torque, not horsepower. Your biggest issue is whether you will keep the EFI or not. You'd probably need a Bigblock throttle body, and a new prom for your computer if you stay with the efi. If you go carb, you'll need a non computer controlled distributor as well, you'll have to alter you fuel system to work with the carb, plus a way to control the lock-up feature of the torque converter in the trans., and possibly shift points in the trans if its an electric model(4L60E) If you go too big on the camshaft, you'll need a new torque converter all together. You should check the options sticker(inside of trunk, or glove box) to find out what gear ratio you have in your axle. Changing this to a low to mid 3 series gear will improve your acceleration.