Great suggestions from everyone. It seems that the general consensus is to cut back the edges of the Fastrack to fit the MTH girder bridge.
I did some research based on PRR2818's suggestion to use Fastrack transition track. I could purchase two of the Lionel Fastrack transition pieces and two MTH Realtrax transition pieces (there are all 5-inches each). These transition pieces have three pins on one end of the track section, allowing them to join together.
Lionel Fastrack can transition to MTH track over the span of the girder bridge, then transition back to Fastrack.
The cost to do so would be around $40 -- more than the cost of the actual MTH girder bridge ... and I have two of these MTH girder bridges -- YIKES! Although it's an EASY option, it's an expensive one.
I will cut off some of the Fastrack and fit it into the MTH bridge. I can use filler material to level off the Fastrack to match the height of the adjoining pieces, then spruce up the edges with ballast or similar "clean-up" material. Besides, I prefer to see only ONE style of track on my layout (Fastrack) and not "patch" some MTH track into the mix just to cross a bridge (LOL!).
Thanks for all of the wonderful suggestions. OGR forum members are THE BEST!
PS - thanks jmiller320 for the tip about insulating under the Fastrack at the bridge crossover. Don't need any shorts (except for the ones I wear in the summer!).