He wrote Spider-man for a very, very long time. 

There was a dispute over who created Venom though.  It played out in the letters poages of Wizard long ago. 

 Basically, Michelinie said he created him, with some input from editor Salicrup (who switched him from a female to a male) and had plotted the issues leading up to Venom's appearance before McFarlane assigned.  (in which Venom "appears" kinda-someone keeps doing things to Spidey without triggering his Spider-sense but you don't see who it is). 

Peter David corroborated this, saying he spoke with Michelinie at the time about his ideas.  Basically, he said that all McFarlane did was make Venom big and give him a big mouth, and even then these were designs that had already been discussed. That Michelinie took the symbiote, thought of having it a separate character who would bond with someone else and conceived of Eddie Brock and his back story and personalty

I believe Eric Larson wrote back, saying basically David copied and swiped Venom , (and of course the black and white symbiote already existed) and placed it on a crap character with bad motivations and the only reason anyone cared for Venom was because of how Todd drew him

Although, I think it was Larsen who added Venom's fangs and his long tongue-I dom't remember for sure, but I don't think Todd drew him quite that way.

Addressing the question of DC creators working for Marvel in the early sixties...

Here's a link to Mark Evanier's article on creator ghosts that lists a few creators that did uncredited work for Marvel.  I don't know if any were working for both DC and Marvel when the X-Men arrived on the scene, but I wouldn't doubt that someone at DC had helped with a little pencilling or inking on some last minute back-up feature or two.