Did you also say you rebuilt this?

What do the heads on the broken 350 look like?
What have you got for exhaust, cam, carb, intake now on the 305?

" Speed costs money. How fast can you afford to go?"

You can prolly pick up a used running 350 for under $500. That would be the cheapest what to make the 305 go faster...replace it!

the 350 will bolt in, but if you have a single exhaust, the Y-pipe is a bit wider on a 350.