Heater hose routing
 

Hi I have a 1985 GMC Sierra C1500 5.0L 305 engine. I have a question about how the heater hoses are routed on this year truck. When I bout this truck the hoses to and from the heater core were not hooked up. I am wondering where the inlet comes from and where the outlet goes to. There is one big and one small connections on the heater core which is which?? Thanks in advanced!!

The hoses run from a barb on the back of the intake into the heater core, then from the heater core right into the radiator on a nipple that's right underneath of the pressure cap. You can see the one long hose on the passenger wheel well liner underneath of the intake snorkel tube.

I've also seen mods where people take a new style SBC water pump off of a 96-98 truck and finagle the bypass through the extra nipples on the water pump, but I'm not 100% sure on how that's done. I've heard things can get real bad if it's not routed correctly through the intake.

Heater core connections
 

Thanks but when it flows to the heater core there are 2 different size tunes coming out of the heater core which is the outlet and which is the inlet

I don't think it matters so much, but if I recall correctly, the barb on the back of the intake goes to the upper port on the heater core, and the lower smaller tube goes to the barb on the radiator. So if I recall right, it flows in the top and out of the bottom barbs of the heater core.