You could save money here to and buy the conversion kit.
Kit No 5428 with front differential for your on-road car, or Kit No 5429 without front differential for your off-road car.
The instructions are an exploded view with almost no text that shows how to assemble it and where you should drill the new holes in your old chassis.

I am going to use this kit to build a Carlsson Viking 4WS 4WD GB when I get all the parts.