The Z is a unibody car. That is to say, the body is the structure, without a separate frame underneath. The roof is part of that structure. If you choose one of our roadster conversions or topless rebody kits, you need to reinforce the chassis.
I sell a clean, all-in-the-cockpit reinforcing kit. This is a minimum level kit and if you driving style or engine power dictates, you will want to consider more extensive reinforcing.
Should you reinforce before or after removing the roof? I find it easier to do afterward, because it is a lot easier to move around inside the car without the roof in the way. Yes, the car may sag a little when you remove the roof. This is the main reason most folks want to reinforce before roof removal. However, if you remove the roof first it is very easy to put a floor jack under the pan to "un-sag" the car while you weld in the reinforcing.
One caveat, though: Particularly in the area around the top of the windshield, there seems to be some residual high-gravity effect that will attract your scalp to the rough edges where you removed the roof. I'd recommed you at least rough-install the header cap after removing the roof and before beginning work inside the car. Voice of experience...too many times to count.
Consult the Velo Rossa™ installation manual for more reinforcing information.