You've got your hands full...
The girder beam being spliced at the existing columns is a bad deal from the start. The floor would have to be torn out and a footer dug to accomodate the weight of the new post.
I can't tell you what to do about the beam. The liability of telling you that you can do something, then if it fails, is more risk than any of us should take.
This is major structural remodeling. I can't recommend a homeowner to do anything this big on their own.
I can suggest this. The steel plates will hold, but the weight will be on the bolts, not a good situation. If the bolts shear, your house will settle hard and sudden until it finds something to stop it. I would go find an engineer, or at least a licensed and insured contractor. An insured contractor will have to stand good for any damages caused by the job he does. Most likely, he will have to build temporary support walls and replace the beam. Not a job for the average homeowner. Considering the cost of changing the structure of your home. It seems excessive to move a post only 5 ft.