In my opinion,
Regardless of how fantastic the additive package in Del Vac is (and it is), 15W-40 is FAR to thick for a 1948 Chevrolet engine.
Viscosity is just as important as any other oil attribute.
No matter how good the additives are, the oil has to get where it's needed in sufficient quantity first and foremost. Especially at start-up.
Using a thicker oil in a low pressure Chev six may raise the oil pressure, but it will actually reduce the amount of lubrication the pistons, cylinder walls, camshaft, lifters and anything else relying on oil thrown by the dippers get.