[Accelo]

If the '70 needs any head work, you could wake it up with a set of Vortecs.
I agree. I would build the 70 block as it may have 4 bolt mains. And if your lucky the higher nickel block. Although they seem to be rare.
The higher nickel block do wear much better than the standard block. A 100,000 mile higher nickel block typically has .004 to .005 cylinder taper where as the standard block may have twice that. But the typical mileage enthusiasts drive their 50 year old trucks it would hardly be of any consequence.
I would also use the Vortec heads. More power, better mileage, and valves and seats designed for unleaded. They do require a matching Vortec intake.
IMHO a rebuild 283 has almost zero resale value. Where as a properly built 350 does.

Considering a modern GM 3.6-liter V6 has 335 horsepower, over double the HP of the 283 I would go with the bigger 350. With some luck it may get close to the HP of the modern V6.