Speed costs money $$$...How fast do you want to go?
Any truck (leaf or coil) rear will have trouble hooking up due the the lack of weight directly over the rear axle....
An LQ4 at 200K is just run in...I would change the cam, freshen up the rings and bearings, cam chain and oil pump, service the heads and install valve springs to suit the cam.
You didnt mention transmission? 4L80E makes sense to me...
Just my 2 cents worth
