Found my screen didn't come anywhere close to fitting at first - the gap between the screen and the top of the roll cage was simply laughable, and like yours, worse on the offside.
In the end I couldn't see any way of getting my screen to fit without altering the channel for the screen, and I also found raising the front edge of the dash by about 10mm very beneficial.
The fit of my screen is now pretty good, and the fit between the dash and front panel is just about okay.
I did at first think my approach of lifting the front edge was a bit unorthodox, but looking through the archives of this forum I noted some of the early builders did a similar thing.
For reference, I found ebay the cheapest source of proper windscreen bonding adhesive.