Before you go jacking and installing support columns a few things come to mind.
Check the roof ridge from above. If the ridge line looks straight and even you don't want to raise one side and leave a step at the center of the roof. You will probably find that the sections are lagged or spiked together across the roof ridge, under the ridge cap. If this is the case you don't want to jack one side without the other as you could tear the roof sections apart causing considerable damage and a possible view of the sky
Homes built at that time were generally installed with support under the main frame I-Beams. They did not do much perimeter or centerline blocking. I would start at the bottom and work up. Check to see if the home is out of level, especially in the area that you see the problem. The way double section homes were installed in the 70's was most often to get one side level and then rack and force the other side to match at the floor line, so the home may very well have been out of square since it was installed.
A few additional piers along the center line or a full relieving might correct your problem.
Here are a few online resources that may come in handy-
Is Your Manufactured Home Unlevel?
How Level Is Level?
The Manufactured Home Releveling Kit
If it turns out that you do need additional support under the center line from inside do you really want a few steel posts on the middle of your home?
I would consider beefing up the interior ridge beam and adding support at the ends to maintain the open span. Perhaps two 2x8s the length of the span nailed together and stood on edge, with 4x4s or the metal posts under the ends. If it comes to that bring in someone that has experience opening walls for large window spans to figure out the best support needed to maintain the open span and where it is best to put the supports, to be sure the weight is transferred to the foundation system and not straight through the floor boards.