I ran a 351c in my 67 for about 10 years. No mods needed. The longtube headers were a real headache though. I had Hookers which at the time were the only ones available for the 2v Aussie heads. They scraped on most driveways and eventually crushed the tubes. Changing plugs is a couple of an hour affair also. I have since swiched to a Paxton supercharged 302 and love it.