I think if you want the safest you need a modern three point belt, and be sure to reinforce the mounting points, not just a bolt through the sheetmetal, w/ a few washers.
If you're more concerned with it looking as stock as possible then I would go with just the lap belt.
I'm planning on putting in airplane-style lap belts, cause I think they look the best. But, I'm also leaving the stock drivetrain, meaning that it'll never go over 50 for an extended period of time, or out on a modern 70 mph highway.
Odds are that you'll never need them. Seems to me that everyone gawking at your cool old truck would probably cause other people to get in a few wrecks!
My $.02
