AFAIK the Spider never made it to radio, though he did make it into a serial cliffhanger. (See the bottom of this page
for more on that.) You can read a couple of the novels for free -- legally -- at the Fifth Imperium
Back when Blackmask Online was still around, they had the first five novels. Things hadn't gotten quite as over-the-top then, and the inspiration from other characters of the same era is obvious, but a "low rent version of the Shadow" isn't the phrase I'd use.
There used to be a Blackmask DVD available for purchase that included all the site content, which also had all the Doc Savage and Shadow novels. Feel free to guess why the website is long down.