While it's true that you won't be able to take an e-commerce module and just put it in an Ãœbercart site and expect it to work, you can still use a lot of the code that doesn't have anything to do with Drupal. In fact, that's where the USPS module came from. Most of the original code that built the XML requests is still pretty much the same.

I don't think I'd worry about a bridge module. It wouldn't really be generic enough, and you'd need to spend as much research and mental effort doing that as you would rewriting the modules.

Product dimensions.
Originally, I had put the length, width, and height in the UPS module because I figured that not all shipping methods required them, and even the UPS module doesn't need them if you choose a standard package type. I also thought that maybe people would want to specify different dimensions for the different shipping methods. UPS boxes are different from FedEx boxes, after all.

I suspect that my reasons are silly, and I'll probably put the dimensions right into the product module. Product weight has been there forever, despite thinking that we'd eventually use it for file downloads.