I bought a new poptop a while back, apparently a copy of this type:

The guy that was supposed to fit it went awol and I've ended up keeping it in the back of the van for ages. But a couple of weeks ago I took my courage in my hands and tried to fit it myself. (Out of an abundance of caution I decided not to cut any holes in the roof until the poptop was opening and closing as expected.)
Then I found that the hinges and brackets for the struts seem designed to work with a flat horizontal surface and don't follow the contours of the roof at all. So question 1 is whether I should build up mounting points for them with body filler or something?
The second issue is that the mounting points are above structural members, which is probably a good thing, but I can't see how to bolt through the roof and attach the nuts to secure them. Is this a problem with me being supplied the wrong fixings?
Do I need to drill right through everything and use longer bolts? Or just use self-tapping screws into the sheet metal of the roof?
Or is it more complex than that? Everything is resting purely on the tintop; close to some support, but no immediate connection to the structural members. That makes me a bit nervous, although I suppose it's not that heavy. Should I be supporting from underneath somehow? It's a bare panel van at the moment, it's all fairly accessible.
The instructions supplied were not very detailed, I'm basically making it up as I go along now. Any advice or insights much appreciated!