His IRS is perfectly fine. There is no point for him swaping to a solid axle for the type of "fun" he has with the car. 3:73's would go excellent with your car. What springs are you going with? 18" wheels will be nice especially with some DR's stuck on the back 2.
Not sure what route you are going with your car. But Modular motors love the gear. So even going to a 4:10 wouldn't be that bad. How are you liking that Tri-Ax?
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"Gripping the wheel his knuckles went white with desire, the wheels of his mustang exploding on the highway like a slug from a .45. True death. 400 horsepower of maximum performance piercing the night, this is Black Sunshine"