I'd be looking above the starter on the rear of the 1-4 cylinderbank. BTW, I see that C6ME appears on the 390 list as well as the 428 list. If you have the heads off, the stock bore of a 390 is 4.05" the bore of a 428 is 4.13". Of coarse previous bore enlargement can cloud the issue. Do you have a view of the flexplate/flywheel? Is it counterweigted? BTW a 428 crank in a 390 block makes a 410 (use 428 balance). I recently saw a 73 F100 Ranger w/ a 410 in it (very nice).

BBT