If the kit off the classic has two vaporizers you shouldn't have any trouble.
If it only has one, you may find it can't keep up the juice required for the 4.6
You could always add to the kit though.
I've seen people run two separate pipes from the tank to two vaporizers each feeding either side of the V8. This seems to work best.
Make sure you check with NGK Technical about your gap settings on LPG as it will put a hell of a strain on your ignition if you have too big of a plug gap.
NGK is on: 01442 281000.