If you choose Tainted (mutations) as Lure of the void, and you choose to pay 200exp to choose Ravaged Body.

Do you reroll any rolls above 74, or can you keep them?

There isn't any reason you'd need to reroll. The limit is only for the initial mutation. That said ravaged body is a horrible choice. As a gm I'd make you roll the mutations in front of the group, and keep everything you roll. At least one the 5 mutations is likely to make your character near unplayable. Heck IMNSHO at least half are bad.

Keep in mind that the Imperium is very anti-mutant. Sure a Rogue Trader has certain liberties which includes having mutant, and xenos henchmen. But taking mutations is going to limit your character in most social situations, and it wouldn't suprise me if at least one of your fellow players didn't start plotting the death of the mutant scum. (Given the setting it would be completely in character.) The navigator is likely the only class that can get away with mutations, but even then ravaged body is pushing it. If you look at the mutant entry in the back. A mutant has ONE mutation. The mutant abomination has 1d5. You are signing up to be heavily mutated even among the mutant under class.

PS- Sure it's a great choice if you are going to "roll" 3-4 characters and cherry pick the best one, but in my group we call that cheating....