From a vintage racing standpoint, this topic has been subject to a fair bit of discussion in recent years. Here in the Northwest, the Society of Vintage Racing Enthusiasts, SOVREN (
Society of Vintage Racing Enthusiasts), regulations are now very clear. Vehicles with windows which may be open, closed or somewhere in the middle are now required to be fully open. On the other hand, vehicles with 'fixed' windows such as the GT40 and Lola T70 may run with the windows in place, just as they have for the last 40 or so years. As for other race organizations, I'm not in a position to comment as I'm not familiar with their requirements.
Which is what it all comes down to -- familiarity with the rules of the group(s) you intend to run with. Once you decide with whom you're going to race, get in touch with a member of that group and ask the question or, better yet, pick up a copy of their rulebook and start reading.
From the perspective of a corner worker with more than a few years of experience, I'd rather see your 40 running with windows. That's how the cars were raced in the day and just exactly what I'd like to see the next time I'm on the turns. If, perish the thought, something nasty should happen to someone in a car with fixed windows that would require 'removing' the window to get the driver out, you can bet next months salary that my boot would be through that side window faster than you could say 'New York minute.' When it comes down to it, looking after our drivers is a wee bit more important than the plexiglass, right?
Best,
T.