I guess you could plug them. The issue you may have is removing the 40 year old fittings. I would recommend doing the loop back. You can shorten the hose if it bothers you.
Yes, you can. https://www.amazon.com/Dorman-02250-EPDM-Coolant-Bypass/dp/B00Z7MM0IG Use a hose clamp.
Here's a complete setup: http://www.v8buick.com/index.php?th...ter-evaporator-and-inner-heater-boxes.361722/