Questions on the radiators overflow resovoir.

Well, i got the dieseling figured out, had to adjust the weber carb, fixed it right away, thanks for that info and well i'm back again. This time i'm over heating, yesterday i noticed it running a little warm and figured was just needing a thermostat, no problem... well after driving for an hour ( highway ) i ended up pulling over to find the overflow reovoir had a pull out top wit a split connector running into the overflow,( A split 't' with a hard plastic hose from the cap into the overflow and splitting at the top) now whoever owned it before me had just an open hose running form one side of the overflow, ( opposite end that's ran into the radiator) it was just a 90 degree piece tubing, had steam and coolant gushing out of there, so i picked up some coolant and a line stop and capped this off. it's still overheating, but i'm moving to the thermostat then the water pump. but i would like some more info on the overflow resovoir and if the line i capped off may run somewhere else or if i did the right thing will take pics and post them sometime this evening.
Thank you guys for all the help so far, very glad i joined this group lots helpfull advice tips/hints in only few days i been on here asking!

The overflow reservoir is a "T" connection on top. One side faces to the radiator and on the other end of the horizontal connection is the short "L" shape hose to spill out which shouldn't be capped off. Of course the vertical connection is into the reservoir.

The other things causing the overflow is a bad thermostat, a clogged radiator or installed in reversed thermostat.

How is the oil color when checking the dipstick?
How new is the radiator?

So i should uncap it and put the hose that was on back on there??
The oil isn't good, as in it needs changed, but no murkyness or discoloration, not sure on age of the radiator, but i plan on flushing it. all you stated above is what i've planned on doing the next few days, checking inverted thermostat ( or replacing very cheap so prob just replace it) i'm flushing the radiator when i change the thermostat, replacing the top and bottom radiator hoses. then if from there it's still overheating i'm going on a new water pump, and then god forbid a new radiator. But thanks for veryfing that hose for me.

It is the "T" shape as you described yes one hose from the radiator, the other facing towards the firewall was the end with the angled piece of hose, so i should take the cap off and replace the hose ( just making sure i understand ) and if it is this way could you maybe enlighten me as to why it can't be capped off?

Well, i took the truck down and back putting gas in, enough that my thermostat should have opened on that drive. well parking it at my house the temp gauge running a little high nothing crazy maybe little under 3/4 mark. Feeling the radiators top hose to the thermostat it had no water pressure, it was easy to close shut with two fingers, as well as the bottom, so possible bad radiator as well as a water pump?
Going to flush the radiator tomorrow and see what that does as well as a new thermostat or check see if it's in backwards.