Why? Most SDRs will handle at least 2MHz of bandwidth, and some do 10MHz. You can't do anything near that looking at a 455KHz IF from an external radio. If you're doing spectrum analysis, using the SDR on its own will let you look at much more spectrum simultaneously than looking at an 455KHz IF tap from another radio. Unless you're trying to tune weak signals the SDR can't capture natively, the exercise is pointless.
Thanks Jon - That's the conclusion I'm coming to. I found on Amazon a Mohoo FY2202S 2MHz Dual Channel DDS Function Signal Generator Sine Square Wave Sweep Counter for $75 and a Hantek HT6022BE20Mhz 6022be PC Based USB Digital Storage Oscilloscope, 20 MHz Bandwidth for $61. These are not advanced test equipment and reviews are mixed but as far as I can tell they would do the job. Thoughts?