Been working on a few of them a friend has.
VERY floppy. (and I come from SN95 mustangs), the ergonomics are either gonna be good or horrible depending on your preferences. Its kinda legs flat out in front of you seating position.
The exhaust routing is indeed awkward. GM didnt seem to be thinking about routing the exhaust when designing the floorpan.
Later IROCs have the swaybar and springs you want. To get better then the factory IROC springs, you will most likely be looking for a Ground Control setup.
I hear there is some chassis steering brace called a "wonderbar" that you want to get. (I think my friends car has it, dont know much on how much it helps)
350 is a straight drop in. I think the computers even cross exchange (may need to be chipped, not positive) If buying one to LS swap, go ahead and get a 4th gen as you will spend lots on the parts (look at car-part for 4th gen T56 prices)
Manual trans cars are somewhat rare. you will find that 9/10 seem to be automatics (at least). this is their least endearing quality to me.
The factory 350's ONLY came with automatic trans's. I dont know if the WC T-5 is up to it for sure, so you may have fun times finding a cheap manual trans to back it up.
Speaking of backing it up. They have weak rear axles. Aparrently, if you go much over 400hp/tq they will break pretty easily. Upgrades arent cheap either. Seems the price to upgrade the rear can be >$1200 (or get a supply of factory rears and do the blow n' swap) (if you know a cheap solution or have a dana 3rd gen rear for sale, I might just have a buyer for you)
Good tires in the stock 16" size are limited as well.
Again, these are impressions from helping a friend with his. He isnt that serious of a performance guy, but I have been trying to learn this car to help him.