TMI sells a seat foam kit that will fit on GT seat frames from 01 and up cars and allow the use of their skins. They're usually really cheap from a junkyard if you are interested in that route.
Unless they've been updated, Walbro 255's are not E85 compatible. The pumps will prematurely die. While they do live, I would be surprised if they supported beyond maybe 750whp.
I suspect you'd max out the injectors around 850whp.
This is the way. Add a shifter of your liking to the list and you'll be set on the mods.
I wouldn't mess with the seals on the engine unless you were experiencing leaks. Don't waste money on control arms for a cruiser. Replace bushings and ball-joints as needed. Sway-bars are unnecessary...
Any change in headroom with these, for better or worse?
6'4" tall guy problems, here. I've had side mounted, fixed back seats sunk virtually to the floor for years now to get more headroom but I'm getting tired of climbing in and out of so much bolstering.