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It's a 4.6 dressed up with late 3.9 acessories, 14CUX, gerotor pump, serp belts, etc.

All Rover V8 accessories can be swapped out between generations, save for the spacer to mount GEMS and later engines onto older cars.

 

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Off topic, but what type of ignition upgrades are available? What kind of performance increase would you expect from an upgrade?

Buick-Petronix dizzy and an MSD 6A control unit\coil are what I'd think of first.