What you need to understand is that you are trying to make this pan into something that it is not. To understand the function of this piece, you need to understand stove technology when it was made. To use this piece as intended, one must remove two stove eyes and an intermediate piece from the working surface of a wood or coal burning stove. The outside frame of this pan supports it on the stove top, while the gate marked bottom hangs down into the firebox of the stove. The gate mark, in this case, in no way impedes the function of this pan. The pan was not intended to be a griddle sat flatly on its bottom for use and placed on the top of the stove or in the oven.
Bottom gated pieces typically were not ground, and that is why they do not sit flatly. If you grind it yourself, the piece is strictly a “user” piece that has no collector value. But what is the collector value of this piece to begin with? I have no problem finding these for sale for $10, and there are a lot of them out there. Modified cast iron cookware is easy to spot. Just look at all the broken pans up for sale on ebay. Be aware if you try to re-purpose this pan, you may end up ruining it. Gate marks are tricky areas on cookware. I have seen many pieces where sand or some other inclusion creates a hole in the gate mark. This spot may not be visible or may appear as a small casting flaw, but with use, rust removal, or grinding, a hole can inevitably develop. I cleaned a teapot so well once I found a toothpick sized hole right next to the gate mark. That was a true moment of disappointment in my cast iron cleaning career. Finding issues like this during cleaning are bad enough. I for one do not recommend modifying old cookware. Cast iron cookware is not readily repairable, and its use, let alone its value can be significantly impacted by repairs.
One other aspect to consider before using a long pan as a stove top griddle is the fact that most of them are quite thinly cast. Exposing it to the direct heat of a stove burner gives it the opportunity to warp or distort. The point I’m trying to make is that good ideas are always well intended. The end result, however, is not always in favor of the thinker - or the tinkerer in this case.
Personally, I’m looking for a wood burning stove that allows me to use one of these pans as intended. There are certain cooking experiences that cannot be duplicated on modern stove tops. The “odorless” skillets and griddles are other great examples.
