Just to avoid confusion, there was no '68 351. '69 was the first year. The differences between the 2V and 4V versions are pistons, intake, carb, plus standard dual exhaust for the 4V. That's it.
ALL 351Ws came with beefed-up block, crank and rods compared to the 289/302 engines, so there was really no need for a "heavy-duty" version.
Don't use a double-pumper carb on a car has an automatic and/or "highway" gears. On a stock engine, 650cfm (or even a 600) will be fine, but you can definitely get by with a properly set-up vacuum secondary 750, and not grow out of it should you upgrade cam, heads, etc. later.
Edelbrock Performer RPM and Weiand Stealth are good intakes for power while maintaining driveability. Edelbrock Performer or stock Ford intake will make it easier to fit everything under a stock hood, but can become restrictive with further upgrades. If you use a stock Ford intake, be sure to get the spacer that goes between the carb and intake.