Having no real idea about how a 76mm works. Is the 18 tons is fixed in place (not supposed to move when the gun fires) and how is the recoil damped?
EDIT: Just read the original thread, and under the conditions you state I dont think its possible to acieve what you want.
The problem is unless you have the same momentum then you won't have the same recoil. It would be possible to use a larger charge snd 'safe' mass but you arent going to get anything like the recoil you want.
The only reason I say this is impossible (I hate to be defeatist :() is that its stated that it still needs to be able to fire live rounds, so no major alterations can be made, meaning faking the recoil is not a possibility.