Standard router inlay kits rely on different size guides to cut the inside (tabletop) and outside (insert).
One template (in your case, a little bigger than 51"x7") will guide for both the tabletop and the insert cut.
If you wanted to get fancy, you could make a second template that would guide for the smaller hole, but since there's math involved, I wouldn't bother. Plus, you'd have to be perfectly accurate to center the two.
I would use a router guide collar to cut to a slight offset within the original tabletop cutout. (If the guide is deeper than you want the first recess, you'll have to shim the router base with something.) That'll take care of the stepped hole through the rest of the tabletop. Square the corners if that's important to you.
Now that you know what size the smaller (lower) hole in the tabletop is, you can cut your insert to size. You can go nuts with accuracy, or make it slightly loose... if loose, the top will hold it in fine. The goal is to get the underside of the insert dead flat.
My best idea would be to use a bearing guided rabbeting bit (exact size would be tricky), though you could also use a straight bit on a router table with a fence. A bearing guided wing cutter from the top side would work, but would be tough to keep perfectly accurate. You could also pass the insert through a table saw.
Last tip: make your template way oversized so you'll have the ability to clamp it to the table without getting in the way of the router.