I'll chime in:
About 500 or 600hp would be a about as much as you'd need for a street car, anymore than that and your bordering on it being a racecar IMO.
Getting 500 or 600 Hp to hook up is gonna be probelm unless your narrowing and tubbing it out for some wider tires, cooling is gonna be another concern.
I don't know what rearend you got in there but if your going with a big iron, you need to upgrade to either a 12 bolt, 9 inch, or dana 60 housing.
10 bolts are for small blocks period.
That TH400 you got already should handle the chores of shifting with only minimal upgrades.
JMO>Cp