The castors looked suspect to me, too, and I SURE wouldn't wanna get a corner of that thing on me! Also, if you are welding something toward the middle of the table, aren't you apt to get some warpage withoue a lengthwise stiffener under it? 3/4" is a heck of a lot of steel, but heat can do funny things to it, too.
Great use of a road plate, though, and I sure wish I had it in my shop! As far as the width - heck, leave it, if it ain't eating anything; there'll be times you'd pay good money to have it back if you took it off or cut into it. If you don't have to use it, you don't have to!