78Z/250GTO

sure could use your advice being new to the v8 scene. I beleive I have a 305 350 turbo nissan 1987 200sx with a r200 411 diff in it.Runs not to bad but with the stock 411 gearing it sucks on the highway. Toyed with a different diff and will change it if I can fin one with the 3.36 gears in it but i was also toying with a 700r but my tranny is good how about a gear overdrive unit. Buddy with a used 700r out of a 91 pickup for 500 dollars. Found one on craigs list when i suggested sending it cod and i would pay the guy says send him a draft with shipping in advance. I can't do that . The 200r guy says if i put the overdrive in then the shaft will be to short with a bad angle? what the heck do you think about all this. on a limited budget. thanks in advance