So, I have a nice race subframe that's been sitting around my place for ages and I'd like to instal it, but need to be able to keep the stock undertray (it's a street bike w/single seat tail). Anyone have experience fitting one with the stock undertray? Anything that I need to look out for or is it just a big pain to fit?

The rear of the undertray will usually need chopping, it's just a case of seeing how it will fit.

Your subframe depending on make might have the mounts at the front to fit the undertray then just secure the back with cable ties or make up some mounts.

Thanks DM, that's the information I was looking for.