With those tyres the tailshaft should be doing 2270rpm at 100km/h, so in 4th with locked converter I get 1580rpm with a 0.696:1 4th. 3.08 will change that to 1750rpm. If ypu are running up around 1900 the converter musn't be locking. Is the engine standard? Exhaust? Do you have the right speed sensor and correct drive gear for the speedo cable?

A standard EFI 5.0L should easily cruise a HZ wagon at 100km/h at around 1600rpm but if the cam has changed it might simply need a new converter and the Memcal changed to suit (tune and torque converter lockup). If cam is too big it might need a lower diff.