Don't mean to be an alarmist but a loud cracking sound could be a weld on a stressed chassis component breaking! Did the sound repeat or was it a one time occurrence? Usually stones into fiberglass are very stealthy in their mission. We don't necessarily hear them but we see their work as stars in our fiberglass sometime later.
You might want to check things like motor mounts that might loosen up and ride up on a couple of threads and then settle down again into their correct locations accompanied by a loud sound. Did the noise occur under power, decelerating or coasting?
Suspension components like shock mounts might break loose under the right conditions (very unusual) but they would create a loud cracking sound when they let go. You should notice it immediately however, in the handling of the car.
If you can you should either provide some crash protection, like a skid pad, for the
oil pan or better build a lower profile pan that doesn't get as close to the ground. It's not hard to do your own pan. Look at some of the pictures of good comp pans from builders like Avaid who did the original pans for Shelby.
Even if you don't have access to welding equipment you can still draw up a new pan with some of the low cost to free 2D drafting software available today. Take your drawings to the local sheet metal shop buy the sheetmetal and farm out the welding job to a local welding shop. It's smart and its good insurance!
By the way, ditto Trularin's comments about the windscreen. Do a search on the site for broken windshields and you will find a wealth of information. They are tender and expensive so be protective.
Hope your mystery noise turns out to be no big deal.
Ed