Venom as it pertains to engine modifications is a registered trademark of the OMC (outboard marine corporation). Really. Not to mention Chrysler used it on a show car, so they likely have the "Automobile" rights. Marvel Comics owns the trademark for publications. I looked into registering it for way back in 91/92 time frame for the Club. So if you wanted to put Venom on your car, no trademarks lawyers form OMC would "likely" come after you.
With that said, most realize that Venom 500, 600, etc. are what HMS uses to denote his cars, so to that extent is considered his legel "trade-dress". He may be willing to sell you a sticker (you can get one made by any good sign shop, all they need is a scannable picture). It would sort of be like free advertising for HMS, so what's teh harm.... but he may not like the idea of a person doing the mods and putting his stickers on. Really his call.
As for it being done by him, he usually puts a little plaque in the car (on the console by the shifter) denoting the "Venom #". So confusion would be unlikely. He does not tie the "Venom #" to a VIN directly, it appears his records use the customers name. A fellow contacted me who bought a Venom 550 car, but the plaque was gone, and HMS needed the name of the person who had the work originally done before they would send him a new plaque.