Hello I’m swapping to a tbi into my 3rd gen and was wondering if this line runs to the heater could I just plug it?

this engine came out of my 87 Silverado

 

It is for the heater hose. Why don't you just use it as a heater hose? Are you deleting the heat?

 

when I bought the car it had a block off plate on the firewall so I was just going to leave the ac and heat off

 

Probably more work than it's worth to put it back in but it is nice to at least have defrost.
As far as your hoses, I'd suggest running a bypass hose from the intake to the radiator or waterpump. (if you have a port on the wp)
It might not be needed but helps bleed the air out of the coolant system too

 

my water pump does have a hole with a bolt on the top if that’s what you mean otherwise would it just go to the hole on the radiator right below the cap?

 

I don't know about a bolt in it but there is a 5/8 inch port port on some waterpumps.
otherwise yes, the 3/4inch port on the radiator is the return for the heater hose circuit