From the Dorman site; Heater Hose Connectors - 3/4 In. X 3/8 In. X 3/4 In. Tee - Plastic
The vent hose is smaller than the 3/8" stub end, isn't it?
One I found on Amazon. It doesn't give the stub end diameter but in the comments it's listed as excellent for LS conversions.
It is also metal not plastic like the Dorman part. It is also slightly more expensive at $18.24.
https://www.amazon.com/Four-Seasons-...e%2C159&sr=1-1