Isolate the heat so it won’t travel down the bar, full penetration weld, ER-70 Rod, "DO NOT OVER HEAT THE MATERIAL", Polish & radius all welded joints. Yes you will lose HT at welded joint - no biggie - if you keep the heat isolated, the rest of the bar will not be affected. Let the weld cool on it’s own to room temp. After installation, check you bar periodically for stress cracks in the weld affected zone. Personally, I would x-ray the joint but don’t think it is necessary if you are a competent welder. It doesn’t cost me a dime though.
You could re-heat treat the bar which requires a little more involvement with materials, welding and you might have to make a jig to keep the bar from distorting. I think this is a little over kill for any moderate use application - Non Full Race / Abuse