1990 Buick Skylark 3.3 V6 Coolant reserviour has two hoses, one to radiator, one to metal line that crosses front of intake manifold to transmission end of engine. Where does it return into cooling system?

i think you'r talking about the return line from the throtle body to the coolant reservoir.
are you having a cooling system problem?

To mws919 - Had persistant overheating problem. Son changed out motor but different motor wasn't same and old was is gone. At this late date, I'm trying to put things back together! Second engine doesn't have apparant place to attach hose.

what type"emission"/size engin was installed?, is it of the same year?

(depending on the year of the new engine) the metal hoes will either run to the thermostat housing or to a metal hose which connects to the throttle body (in two places) and then to the heater hose (by a quick coupling).

ya, that's what i thought too befree, but when i looked on alldata it only showed a closed thermostat housing" no port/fitting, and the only small coolant lines were for tbi heating.
in less it's some thing to do with emissions, i donn't know right off the top of my head.
i'll have to look into it a bit more.

A third option could be that it screws into the heater hose connector (after going along the valve cover), just past the throttle body--toward the firewall.

it's the coolant bypass tube and it screws into the back of the manifold.
look at the back of the intake manifold, about center, you should see the threaded hole it goes into.

is there water leaking out of the intake manifold when you fill the radiator with water (since you currently don't have the hose connected)?