Just stumbled upon this looking for other info... but have some thoughts so I figured I'd pitch in.

MPFI is a more complicated (and expensive) fuel injection conversion. If you went with with a throttle body injection conversion it would be much more cost effective... You might gain a MPG or 2, maybe more depending on how well tuned your carb is now. You could utilize a fully tunable ECM system like MegaSquirt. GM throttle bodies can be had at junk yards all day long... if you're lucky enough to find a 88-95 GM 454 TBI unit, you can probably get everything for $150 and they flow around 800-850 CFM. A $50 adapter bolts it down to your existing intake manifold. To get to 500 horse you might need more flow, so you could look around for someone scrapping a 4 barrel holley TBI that flows around 1000CFM. If you had a dual plenum intake you could run two TBI units... either way MS can be configured to run it all. Plan on spending another 500-700 on the computer, relays, sensors, wiring, etc...

The MS systems are fully tunable and adaptable to any kind of sensors, but ready to work with GM sensors out of the box. Your main advantages will be better reliability, slightly better mileage and better drivability (better cold starts, etc..). The biggest advantage is tuneability. Once you understand the system, it does tune easier than carbs IMHO, and whats more is that it'll tell you when its tuned perfect.

Food for thought if you're serious about this, but there is a learning curve involved. I'm in the middle of mega squirting my 81 turbo 301 pontiac right now.