Hi Neighbor. I looked it up and the oil pressure at 2000 rpm for a 1970 -79 250 i6 is supposed to be 40 psi. Are you reading it at the sender with a mechanical gauge or using the truck's gauge? There is the possiblity that the truck's gauge could be a bit off. However, it seems pretty accurate after all these years on my 59 1 ton (292).
As for blow by, well, bottom line, its probably rings. Since we don't get all that much cold weather you could try a heavier straight weight oil. I use plain old 30w heavy duty in the 292. A multigrade would be easier to blow.
