Got a 1962 Lancer Gt. Original Pushbutton tranny. Had tranny rebuilt by a pro. Now the problem I`m having is when I am in park, there seems to be a load on the engine. (Not running freelylower idle speed)When I take park lever off I start to go in reverse without pushing the reverse button. When I push Drive engine frees up and then drops into gear like it should and idle goes back down while I hold the brake on. What is out of adjustment here? Park cable (back of tranny) or gear selector wheel (front of tranny)

Shift cable and possibly button box needs adjustment per the procedure in the factory service manual. Also check both cables for any kinks, fraying or other problems.