Caps are what hold the crank in. Some were 2 bolt, some were four. The four being the stronger version. Too narrow it down, do the same with the head numbers at Mortec. The combination of both numbers will narrow down the application. If you don't have the heads, don't worry. Your block is part of the Chevy small block series. Almost any part for a small block will fit.
If you want to get technical, you could measure the bore. There were only so many bore / stroke combinations. So you could really narrow down the size of your engine.
