It will have 4 bolt mains, and you can check the casting number.... I believe it has a "R" on the front of the block and in the lifter valley. It is also solid around the bottom of the bores...
Two quick checks
R in the stamping
and there are two half dollar sized knobes on the front just below the deck on both sides of where the timing cover bolts up.Most R engines were used as warrenty replacement's in vans and light trucks.Very hard to find.