YES I GOT IT WORKING.
steps:
install ksp 1.8.1
modify the folder inside C:\Program Files\KSP\GameData\Squad\Parts\Resources\LargeTank, modifying LargeTank.cfg, scalling the mass 50.0 and the name to LargeTankHEAVY!.
modify the name of the folder too, so duplicate that folder, and put in the copy folder everything from the old folder, but let the mass 50.0, more details ask me