bjbus,  I must apologize for not answering your post sooner. The two models have different neck radiuses, It's a matter of opinion. The variax 500 has a 10" radius neck and the Variax 300 being a 12" radius. you can fit a variax 500 neck on a variax 300 body, but about swapping out the electronics between the two models. Anything is is possible if you have the cash & skills but why would you want to, Please give more details and be more specific with your posted questions. There are many causes that can create crackling problems including what ever type of cables you are using like the VDI cable or the two options with the 1/4" cables, The VDI or 1/4" connections, The model selector control,  The five position pickup selector switch or the problem might even reside within the CPU and have memory glitch , which sometimes can be corrected by a reflash and a software update. however I am still not sure what your intentions are. keep in touch pertaining to your situation, and post us back with updates.