To answer your original question about the trans, you will need to swap the GM trans in since the V8 came with ZF trans, and the bellhousing will not mate up. Once you swap the trans, you will need to swap the rear differential since the rear differential gearing is completely different in a V8 and I6. You will also need to get a front differential to match the rear differential gearing.

The X has dropped in price that you can buy a fully running high mileage X for under 3 grand these days.

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2006 Infiniti G35
2001 BMW 3.0I E53 X5 Build date 08/2000 SOLD
Lotus Europa 1970 Destroyed by fire
Lotus Europa 1970 S2 Renault Powered
Lotus Type 52 1970 Twincam Webers Powered
PORSCHE 911 Targa 1982 The Garage Queen
Audi Avant donated to Kars for Kids
BMW 525IT Sold
Audi 4000CS Quattro Sold
Jensen Healey Lotus Powered Sold
Opel 1900 Sold
Triumph Spitfire 1971 Sold
Triumph Spitfire 1968 Sold
Plymouth "Cuda" 340 Six pack SOLD