One thing to consider is the added weight of the big block, and all the mods needed to get it in.
I have a 370hp 350ci in my 4900lbs car with an auto and it's pretty quick. but with the gears you said you'd be running it should be very quick off the line.
I'd just build up the 350. with good heads, headers and a good intake mached with the right cam and carb, 350 can put out tons of power, you could even stroke it to 383ci to get more torque.
I'm a chemisty man and some times little thing's just bug me Nitrous Oxide is N2O