Maybe? For sure, as the cross members were attached with rivets, not with bolts either on top of the frame rail, on the side, or on the bottom. The three bolts shown in your photo have no V portion of their shanks directly beneath the heads so when you grind off the heads there will be nothing left to hold the bolts or the cross member in place other than the remaining bolts on the sides of the frame (and presumably the bottom). If you cannot use the proper countersunk head rivets, at least use screws with flat countersunk heads