use www.mortec.com to get the info for the block and heads. 
On the heads, pull the valve covers to get the casting numbers
On the block, the casting number will be on the back just above where the bell housing mounts.

Gen V blocks do not have a fuel pump mounting boss on them anywhere. There's an easy visual indication for ya.

Some Gen VI blocks do have the fuel boss, some don't. But I doubt it's a Gen IV...that's when you start getting into the Vortecs.

Also, check the block sequence code on the front too. That will tell you which factory the block was cast at, and the month and date it was cast. The 3 letter code after the month and date will tell you the year and what type of engine it originally was. The block sequence code is located either on a machined surface just above the timing cover, or some have it on a machined surface on the block deck right in front of the passenger side cylinder head.

If it's a 78 block & heads, it's a Gen IV, which I find to be the most commonly available block.

Gen IV was from 70-90 I believe.

Gen V was from 91-95

Gen VI was from 96-current.

When I called my local Chevy dealerships parts department they explained to me that there is a Gen IV and the "Mark" designation described variations in the Gen IV. Anyone have some better info?

My gen V had the oil pan bolt up under the timing chain cover. So the oil pan was a bit longer with 2 extra holes. That's just one. I just looked it up again, I think the Gen V's started in '93.

my understanding was that chevy had to switch from "Mark" designation to "Gen" because Mark V was already copywrited or some bs like that

That would be Lincoln