As for the engine, I would go with a GM TBI kit, they can be found complete for around $850 or so, and then run a centrifugal blower. As far as lag with a centrifugal, I dont think so, I like them. You could always just up the pulley ratio, or gear ratio to improve when the paxton comes alive.
If your going for a 671, or 871 I would stick to carburation. You'll find the benefits of tuning them with out eprom reprograming very helpful.
Now just because it's a 70 you'll have to either replace the leaf springs with coil, and/or add... ahh.. what do they call them, those damn arms, my brain just stopped working. Otherwise you'll twist the rearend up.