For smaller upgrades to your 305" I would recommend

1. Cylinder heads with flat top pistons to increase your CR a bit.

2. Headers should increase performance a bit. This is cheap and easy and makes your engine sound cooler to

3. If it has a 2port carb replacing that with a 4port would also give you more power, find a cheap used one. This will require a new intake. This could also be done in a couple of days.