Im not sure what "3/4 bore" means. Stock 390 carb bore is .830 already. You can go to .930 or so without much trouble. You will want to open up the idle jet some. .026 or so would be a start. Some do some e-tube mods. Some shorten and cut a taper on the part sticking into the venturi. \This taper must point a certain way and only involve a max of about 85% of the end with the rest remaining flat. (Poor description, I know) I dont enlarge the holes into the side of the etube. Some do. Depends on many factors on how the carb is tuned as far as work behind the welch plug and low speed circuitry mods. Some polish and enlarge the inside of the e-tube. I may polish but rarely enlarge. Some enlarge or drill additional holes in the aluminum tube portion of the carb the jet is screwed into to increase potential fuel to the jet. (another poor description) Being as this is going on a small block, not sure the fuel "increase" mods will be needed. I have little expereince in putting them on small blocks so I dont know what is truley needed. Good luck.
