1. You don't have to flush, just drain as much as you can out and then fill it up. . .3.7Qt's IIRC. You should use 75W90 as that's what is recommended. Our trannies are fickle as it is. I haven't tried shockproof so I can't tell you how well it performs. I put in 75W90NS in the tranny and 75W90 in the rear diff.
2. Most people go to a colder plug when modifying their engine with TBE, uppipe, etc. The colder plug prevents detonation by not heating up as much and igniting early. The NGK part number for 1-step colder plugs is BKR7E IIRC.