Parts of my truck are 12-16 ga, quite heavy compared to newer vehicles. The floor is heavy enough it almost killed my nibbler, has to be in the 10-12 range. The dash is 14-16. VERY nice to handle compared to really thin sheet on the body. What I did before I approached it was to take the areas of the patch and dash I trimmed off and work on getting them welded up first. Same metal, same thickness, etc. Finding some other sheet similar thickness would be the next best thing. Don't be afraid of a little hammer/dolly work as you go.