Whats the difference, and do long blocks fit into a fbody without modifications?

I'm looking at crate motors and there are 2 options, with the long blocks costing a bit more.

Thanks
Brian

 

Short block is the block, crank, rods and pistons all assembled.

Long block is the short block with heads bolted on.

 

Brian,
Depending on who yous peak with you will get different answers.

My definition of a short block is the rotating assembly installed into the block, but none of the valve train components. So crank, rods, pistons, bearings, rings all installed in the block. That is a short block.

Long block consists of the short block plus all of valve train, covers, gaskets, heads and intake. Some would call that a complete engine, which I guess it is, but that is deffinition of a long block.