Necromunda rules with
40K regulars ?
I think that was called 2nd edition.
Although I admit, it would be fun to try and play space marines with all the added skills.
I'd just use
40K 2nd edition point costs to buy 'gangs' and then use necromunda missions and advancement rules.
Then I would play a gensetealer cult force and laugh as my genestealers cut marine armor to ribbons