Transmission ratios become more important when engine power bans get narrow. The narrower the power ban the closer the ratios need to be, especially the last shift. If the spread between the last two gears drops the engine to a point that the engine is not generating enough horse power to overcome the aero load at that speed you will not go faster, again this is assuming that you have some idea of what the aero load is at your target top speed and that your engine is sufficiently powerful enough to attain that speed. When using big engines like Dave has with very broad power bans transmissions like TH400s and Powerglides can be used because when the last shift is made the engine is not brought down below the power required to maintain the shift speed. Looking at the additional information that Dave is providing it looks like his problems are more engine related than transmission related and once he gets the engine happy it will probably pull right thru once in high gear (again assuming that his final ratio is not to optimistic).
I have seen a number of cars set records using high gear only, they were on the long course, had good torque producing enginies and had pretty good push trucks but it can be done without a trans. I have related this story several times but back in 02 or 03 the no. 302 roadster, runs XXO/FR has a strong GMC six and likes nitro set their record at around 204-5 and broke the trans, but they wanted their dad to make a run so they took the guts out of the tranny, locked it in high gear and pushed him off, it grunted and groaned but ended up reseting the record at 208, high gear only.
This isn't a dark science, it is knowing the engine power curve, the loads that the car will see at the desired speed, aero and rolling, and then establishing the ratios starting from the top gear going backwards.
Rex