Dave,
I ran some numbers using your ratios and several things that you might think about. At 7500 rpm in second you should be going around 182 mph so when you shift into high the engine should pull down to 5050 rpm and you said it goes down to 4700 rpm, this may be simply the speed at which the 400 shifts. It takes so long that the car actually slows down before top gear is engaged. You might also think about turning the engine tighter in second gear, if you went to 8000 rpm your engine should then be at 5400 in top gear and maybe this is closer to your max torque range and the engine will continue to pull. A good big block should easily be able to go 8000 even 8500 in second. At 8500 you would be going 206 mph and the engine would pull down to 5750 rpm going into top gear.
As Doug and Sum say the real solution is more and closer ratios. The general rule of thumb on picking ratios is that each step should be about 1/2 of the previous step. Look at Doug's ratios, the difference is .55 first to second, .25 second to third, .17 third to forth and .1 forth to fifth. Very good selection, it mostly has to do with aero drag being a square function.
Rex