What is this 1 1/2" you speak of? All the interior doors I have messed with over the years were 1 3/8" thick, but what's an 1/8"" amount friends?
I am guessing you are talking about hollow core doors? If so, you should have no trouble cuttings them down to 78". The edge frame stick most probably will not be wide enough to allow for it. When you cut away 2" you will remove the end frame piece and expose the hollow. There may be large cardboard honey comb inside and it may be in the way. Use a sharp chisel and cut it free from both skins and push it back into the cavity. If you have a table saw you can easily saw the face skin off both sides of the stick you cut off and reuse it. Or you can make a new one. Put wood glue on both skins inside the opening, slide in the stick and clamp it nice and flat. If you are not talking about hollow core doors, Never Mind.
One thing to watch out for is Corrugated Fasteners. Back in the day, some manufacturers assembled the internal frames with corrugated fasteners before applying the skins. Not so much anymore. But is you hit one, goodbye saw blade. You could do a little exploratory surgery with a chisel before risking a saw blade. Or use a metal detector if you have one.