340+ for the STREET

Net or gross horsepower, street or track motor, the biggest advantage I would say the Magnum engines have is sequential Multiport fuel injection. Even though the stock ECUs aren't all that great, I would imagine having the hardware already on the motor is nice. If you want driveability and mileage, I say go EFI so that you can start your motor in any weather and have it run like a top.

Secondly, an OD transmission is a great idea. If you want a manual, get an A833OD unit. You may want to beef it up depending on how strong your motor is. If you want an automatic and don't want to pay an arm and a leg for a gear vendors, get an A518 or an A500. They're just torqueflites with ODs (727 and 904, respectively) and lockup converters.

If it helps any, that's the route I'm going. I've got a '68 notchback with a 360/904 and 3.23 rear gears and 27 inch tall tires. Gets terrible mileage now (8 mpg city), but that's because it isn't tuned worth a damn. I'm putting an A833 OD in it this winter (and actually properly tuning it as well), and later down the road I plan on putting EFI on it. I drive it every day during the summer, the only reason why I don't drive it in the winter is the defrost doesn't work, and I don't quite trust other drivers in the winter :D