Frame vin question

[LT1 Burb]

Anyone here have any hassle from swapping frames under your cabs. I have a shortbed frame but if I use it it will obviously not match the cab and I don't have a title with the frame, nor do I know where it even came from. I bought the shortbed truck but after doing a vin search I realized it should be a long bed, which it clearly isn't , nor has it been altered. It is for this reason that I was planning on shortening my longbed frame, at least it would match VIN's, or should I not even worry about it.