Kevin,
Yours may not be the best approach to a project like this, but you also don't provide sufficient details about how you shot the source images.
1) What camera & lens combo did you use. We need to know what the resolution of each image is, and what the lens focal length and camera sensor size were.
Longer focal lengths are usually best for this sort of mosaic stitching, but they present their own challenges when being used from a moving aircraft or vehicles. Note that oblique shooting angles exacerbate parallax issues between shots because of widely varying subject distances.
2) How much subject overlap do you have between adjacent shots?
For most panoramic stitching, 30–50% overlap works best, but for mosaic stitching MUCH more is preferred.
3) Did your aircraft course (track) follow the coastline, or was it flown in a straight line? Both would be the best combination (straight flight track following a straight coast line), but not usually possible.
4) Did you use a gyro stabilizer on your camera while shooting, and did you shoot out an open door or window, while keeping the camera and lens inside out of the air stream?
All this points to how sharp, focused, and color accurate your source images are.
5) Are your exposures consistent (along with the lighting) throughout the 189 images?
6) Can your client accept multiple panels of the coast line, rather than a single eight foot wide "perfectly stictched" image?
Your end goal of a long panoramic image of the coastline and its features without stitching artifacts and distortions in seam areas – AND of sufficient quality to be enlarged to 8-feet wide is not unreasonable. However, your approach of shooting 189 images and hoping to stitch them together was probably not your best approach. Parallax differences between shots are going to be your biggest nightmare, and you (or whomever tries to assemble these) may struggle in post production for many days (for only $1,000) and still wind up with only mediocre results. Some might be tempted to use AI imaging tools to "fix" the errors, but this will result in inaccurate representation of the coast pretty much lacking any visual integrity.
You might have been better off using a single exposure of the entire coastline section shot with an ultra-high resolution camera, or using a scanning (slit) aerial camera / service, rather than making things as complicated as you have. Your chosen approach is probably going to be more time consuming, more expensive, and yield lower quality results than you're hoping for. Unfortunately, your desired oblique perspective is likely to make a scanning or slit scan camera approach unworkable, however, because these generally need to align the images/scan speeds based on consistent camera-to-subject distances.
When shooting something new, particularly for a client, it's best to figure out your procedures (planning, equipment, shooting techniques, production, and post production) ahead of time, rather than just assuming that if you shoot lots of images, something can be put together from them. A forum like this is a good place to get help when planning your shoot, rather than after the fact of having made one or more poor choices.
Good luck, and hopefully you'll have a chance to do a re-shoot. Or you might get lucky with one of the post-production magicians who reside here. :-)
Scott Highton
Author, Virtual Reality Photography