if a 350 is putting out 400 at the motor, with a 700r4,and a B&M holeshot 2400, what kind of rwp will be had with 3:73's? what kind of difference if you make the gears smaller to 3:23s? what ways can you increase rwhp?

If you have 400 HP at the crankshaft, even with the alternator, water pump and power steering pump, you can figure on a 700R-4 and 7.625" axle together eating about 22% of that, which is 312 rwhp. Your converter will reduce that, it's looser than stock. Other things to do to increase rwhp include synthetic lube in the rear axle, choosing a shift kit that does not increase the line pressure inside the 700R-4, choosing a tighter torque converter, running an electric water pump, and increasing the engine power.

you lose about 20% of your flywheel horsepower through an Automatic, and about 15% through a Stick. But some things can slightly decrease that loss.

does having a high flow pump or an alternator amp above 120 does that decrease your power even more? Atilla, what do you mean a tighter torque converter?

A stock GM 1500-stall vonverter is much tighter than your B&M Holeshot 2400.

well you cant have a stall that low if your putting out higher numbers?

your cam is what determines what stall speed you can use. Also, your 2400 stall is in a 12" shell. Get a 2400 stall that uses a 10" shell, and it will be tighter above 2400 rpm.

what do you mean by a tighter torque converter? also does having a high flow pump or an alternator amp above 120 does that decrease your power even more?

When cruising on the highway, going fast enough to be above the rpm the converter stalls to, if the converter isn't a lockup type, or if it is a lockup converter that isn't locked, a tight converter only slips about 50 rpm. A loose converter can slip as little as 200 rpm or as much as 1200 rpm. For example, I had a B7M TorkMaster 2000 in a '78 Camaro with a 305/TH350/3.08:1 combo, on the highway the tach was reading 2450 rpm at 60 mph, while my calculator said I would be at 2400 if that trans and converter had had a lockup feature like later automatics do. That was a tight converter. Another example, at the opposite end of the spectrum. I had a '95 Z28 with LT1/4L60E/2.73:1 combo. Running 65 mph with the converter locked gave me 1600 rpm It was a B&M TorkMaster 2400, and I installed a toggle switch in the dash to disable the lockup on command. The first time I flipped it, my rpm went to 2800, and the trans did not downshift.
this isn't the extent of what all I've tried, not by a long shot, but it seems to be the least I can share to best illustrate the point.

I thought of another way to illustrate looseness or tightness. My '95 Z with the stock converter did 54 mph in first gear, at 6200 rpm. With the B&M, it was reaching 6200 at 46 mph.