I have a friend that recently mentioned selling a 400 sb to me. My question is could an '87 IROC-Z handle the torque from a 400 sb (or about how much torque would it take to twist the chasis)? And also how much trouble would it be to take the 305 out and swap it with a 400?

 

(The server didn't take my first reply. Hope it goes better this time so I don't have to type this all over again.)

How difficult the swap is depends to a large degree upon which induction you have now - LG4 CC q-jet (fairly simple), or LB9 TPI (more expensive). The basic CC carb setup will handle the engine, while the TPI will need almost all parts replaced, Miniram probably being the simplest solution.

If LG4, you'll need to replace or modify the air cleaner for dual snorkel, and replace all the exhaust with headers, cat, & cat-back. If LB9, you'll want headers at least. The exhaust should be '86-'90 TPI single cat application parts. The 400's dipstick will be on the driver's side, which for the sake of clearance will require the headers (stock manifolds will block the dipstick tube, besides being very restrictive).

400's are externally balanced, requiring different damper & flexplate. Such flexplates are typically 168 tooth, vs. stock 153 tooth, meaning the stock starter won't work. If you use the 400 parts, you should be okay although sometimes the starter hits 3rd gen headers, typically solved with a ministarter.

Factory 400 heads are terrible for performance use. Seriously consider aftermarket heads. They'll have to be drilled to match the block steam holes, but that's no big deal.