Rep Power: 210

2000 Buick Century Custom. 3.1L V6, 4T65E automatic transmission. 92k miles

Transmission lines link: https://m.autozone.com/ignition-tune...BoCX-QQAvD_BwE

I recently purchased a Trans-go shift kit and Hayden 678 external transmission fluid cooler that I intend to install soon. The Hayden kit came with worm-screw clamps that I will replace with constant tension clamps. I want to run the external cooler in line with the existing cooler (radiator). The current send and return lines are a combination of metal tubing with about one foot of 3/8 rubber hose in between (shown in pic).

The kit include a three inch straight, flared at both ends connector that clips/mounts into the "return" side of radiator/cooler. Rubber hose will run from this connector to input side of external cooler mounted to front of condenser and tightened with clamp.
The option for connecting external cooler's output side back into rereturn to transmission is either:
a) cut 3/8 rubber hose (shown in pic) and use a brass double sided barb connector
b) cut metal tubing and add a brass npt compression (metal tubing) to barb adapter (rubber hose)
c) use a single or double sided flare tool on metal return tubing
d) cut metal return tubing with rotating pipe cutter and use as is without flaring.

Suggestions please.