edit: i never realized how little 6M-GE info there is here. So, anyway,
it should bolt right up as a 5M-GE replacement. If you need EGR for the sake of emissions, use your original 5M-GE intake manifold.
In the case of 1985-1988 Cressidas, they're front sump, and the 6M is mid-sump, so you'll have to re-use your Cressida 5M's oil pan, baffle, pump, etc. Other than that, bolt-up.
And apparently (don't quote me on this), you can even retain your 5M-GE ECU, but if you encounter problems, a 6M one may need to be used.
The 6M-GE is a 3.0L version of the 2.8L 5M-GE. It's got 170hp and 170lb-ft of torque, a healthy increase of 14hp and 5lb-ft for Cressidas. Though the increases seem somewhat low, the torque curve is much nicer with this motor, and you'll definately feel the low end in daily driving.