Ehp4832

I just recently baught a 1991 LX Convertible Mustang that had been sitting for close to 9 years. Everything is original outside of the new plugs and wiring and only has 15K miles on it. The back bumber has heavy set in dirt that I cant seem to get of regardless of what products I try. Does anyone have any suggestions? I also just had the transmission re done and now there seems to be a loss of power at low RPM, I spoke to a mechanic and he said to Put high octane ga and a fuel injector cleaner (Techron) and this should stop the problem. The only thing that confuses me is that I wasn't having this problem a week ago before I had the transmission re-done, just wondering if anyone has any insight on the issue or suggestions.