Well,
The best investment you could probably make is some better flowing exhaust. That way you'll be able to reap the full benefits of any additional changes you make. I would guess that a set of Lightning water jacketed headers would be worth around 200 RPM, which according to the JPC would get you to around 497 HP which in turn should yield 73 mph. Of course to me, a set of lightnings is in the "mega dollar" category at 2k, but you'll be able to use them with whatever you do to the motor.
The other option is to put that BBC on the bottle. That should only cost you about $400 for the Nitrous, plus any other expenditures to bring your fuel and spark systems up to snuff.
Short of that, I guess you could try and score a decent used blower and run a low boost level to pick up some additional HP.
On the Free or close to free tip, perhaps you could adjust your rideplate to give you a slightly looser set and free up a little more hull.
Hope that helps,
Chris www.liquidaddiction.net (http://www.liquidaddiction.net)