tap size for 2nd gen rear bumper reinforcement bolts

I am in the process of replacing the rear bumper on my 06 Tacoma. Everything was rusted as $hit and despite soaking the thing in penetrant for days, I managed to snap a few bolts on the reinforcement brackets.

I have tried soaking them some more, beating them with a hammer, heating them etc and can't get the broken studs out. So I guess I am left with drilling them out and taping new threads.

I have access to a tap set but no idea which size to use. I tried matching the threads but nothing seems right. If anyone has done this already and knows the proper size can you let me know? just trying to save some money here. Will likely replace the truck next year as it's really starting to fall apart after almost 500,000 km

 

I managed to get all the broken ones out except one, which snapped off on both sides of the nut so just have to drill out that one.