I experienced a near identical problem in the mid 60's with a 352 Ford that had misteriously wiped all the main bearings from #3 back AND the back 4 rod bearings.
Cleaned it up well and corrected the shaft, rods, and rings for what should have been a good freshening up.
On startup the rods were starting to get noisy within 15 miles--dropped the pan (what a bear to remove) and inspected the bearings to find the same thing.Every thing from center main back was starving for oil.The pump is in the front on the FE engines.
Out with it and teardown for inspection and found the block cracked from the camshaft down to the #3 main right through the oil gallery.Evidently when it warmed up it leaked enough to starve downstream bearings.The oil pressure was never lowered enough to make the light come on.
Drop the pan and find a way to pressure up the oil system--air or oil and look for a sizable leak.
