IIRC some 283s had a hole in rear camshaft journal to provide oil to the lifters. Later 283s had a groove around the journal to allow constant flow of oil. Also make sure the camshaft bearing in the block is properly lined up with the oil passage. I had a 283 in my 67 Chevelle that took several minutes to get oil flowing out of the pushrods to the rocker arm. I recommend running 5w30 Synthetic oil to get oil pressure quickly to the bearings and top end. A high volume oil pump is recommennded for lighter oil.

1948(10/'48) Chevrolet Loadmaster 1.5 Ton, 235cid, Muncie P211X, 137" WB, Dual Wheel Rear Axle 11,000# GVWR, Chalky Blue

I like giving advice. That way I don't have to fix it.