396 casting #3855961 no oil pressure

[budd01]

so i have a pickup in my shop with a 396, low oil pressure and a couple dead cylinders, the dead cylinders are 2 and 4 so i'm hoping it's just a blown gasket between them, as for the low oil pressure, it starts at 60 and goes down to 5 and the lifters are rattling , i'm thinking it has the wrong cam bearing and main bearing, can someone help me with what the difference is between the early 396 and later one, thanks, lawrence