There is a cap, sorta, it's on the top of the pressure bottle.
For topping off the antifreeze, use that opening. If you're draining the entire system, many of us will pull the upper radiator hose off and fill the majority of the fluid through it. Except for me. I installed a later model radiator with a regular radiator cap stick out of the top. :D It's exactly the same as the older radiator, but has a small neck and radiator cap on it.
If you draining the system, another good tip for filling is, once you've put in as much as you can, prop up the back of the truck so the back of the engine is higher than the front (causing the bubbles in the block to congregate near the back, and then carefully unscrew the temp sensor (located at the top of the back of the head). Don't take it all the way off. Have someone fire up the engine (and be ready to shut it off if anything goes wrong), place a rag over the sensor to protect you if you unscrew it too much and it pops off (you are wearing gloves and safety glasses, yes?), and bleed the air out past the sensor threads. If you don't get all that air out, a pocket could form at the back and cause the engine to overheat.
It's the hexagonal cap in the pic.