You'll need to port match the inlet of the manifold to 75mm. If not, just use a 70mm. Is it "too much"? No. Will it make more than a 70mm? Yes, but nothing huge.
You'd still have to port the inlet on the manifold. I dont think it'll be a cost effective mod, unless you can easily sell your old crap. You NEED longtubes. You have a similar set up as my friend and he's making 323hp/346tq and thats with gay shortys.
Not to agrue with ya eades, but there was a dude in this 94/95 section that gained 4hp OVER a 70mm TB on his bolt on car with stock heads cam, so saying a 70mm is good to 650hp isn't really correct. 70mm matches his inlet best and its fine. I'll be doing a 75mm on mine as recommended by ed curtis.