This question comes up all the time about a 3 speed vs a glide. Ok so here is my experience between the powerglide vs. 3 speed debate. Most of which I am sure you already know. The powerglide being smaller, less internal parts, and smaller reciprocating parts, requires less power to transfer energy through. Typically if you have two cars that are roughly the same ET, one has a glide and one has a 3 speed, the glide will out MPH the 3 speed, but the 3 speed will have better intervals to the 60 and 330. I have many examples of this with friends I race with who have similar running cars as I do, and their MPH is always better than mine, but they can't touch my increments, even some being faster in the 1/8 than me. This proves true with other cars in my race team and friends as well. This isn't a hard fast rule that fits every situation, but that is my experience on the matter.
Now if we are comparing apples to apples, should you switch your car over to a powerglide as it currently sits without changing rear gears, I suspect it would slow it down. How much? that I can't say because everyone's car is different. Some have their combos figured out much better than others. Yours seems to be a good running figured out combination so I would expect the glide to hurt it some. I am not saying it 100% will hurt it, but that is what I suspect and would expect. There are some guys out there with glides that do leave the line hard and lay down some impressive numbers while maintaining the consistency. A lot of it boils down to the torque converter with a powerglide. The torque converter is much more sensitive to a powerglide than a 3 speed due to the glide not having the steeper first gear ratio to help multiply the torque. So if the converter in a glide is off a couple hundred RPM, it will hurt it much more than if a 3 speed converter is off a couple hundred RPM. A lot of other variables come into play such a vehicle weight, wheelbase, rear and front suspension, etc. But you get my drift.
When a customer calls me about a trans and wants to discuss going to a glide or a 3 speed, I usually say, if you want less bottom end, with less risk of traction on marginal to poorly prepped tracks, and you DON'T want to make suspension changes, then the glide would be your better bet. Now if you want to leave the line harder, don't mind making changes at the track if its marginal or poor, then a 3 speed is the answer. For what its worth, our fastest team car, is a door car and runs 4.60s-70s N/A with a 3 speed. Car has been in the winner circle at regular races and is very consistent. Car is 1/8 mile only.
So overall, I say its all in what you want and are trying to accomplish. All of today's transmissions with aftermarket parts can handle big power, and they all can be consistent and reliable. 2 speed or 3 speed.
I have cosidered the powerglide many times in my own car but the cost to switch to a good powerglide to handle the amount of power you are laying down is going to be costly. Plus you need a different shifter, converter, flywheel, crank adapter for flywheel, driveshaft, and need to modify your cross member in the car. I have added it up many times and it is well over $7k to make the switch and possibly be quite disappointed.
For what its worth, I have had my 727 in my car since 2015. It was originally built by Rick Allison for a top sportsman car in 2006 where it saw many passes at high power. I bought it used in 2013 before I was doing transmissions. I put it in my car on 2015. and never opened it up to have it checked before using it. I put 499 runs on it before I took it apart. Only reason I took it apart was because I had a converter failure and it sent a plethora of metal through my transmission. Nothing inside was damaged and everything was re-used. I replaced clutches and steels just because I had it open.
I will post a video and share it on here of me racing another 1971 Demon heads up in 2023. Both dialed 5.46. Both have same cubic inch and overall engine combos. His car is much lighter than mine. I will share the time slip as well. You will be able to see the difference in how both cars leave with the same ET.
My opinion, don't change to the glide unless you want to spend a bunch of money, and re-learn your whole setup. Make your 904 bulletproof and you'll be good. Many super stockers are running pro flites which are all 904 internal parts with 727 case. They can handle the power and they are going mid 8s.
Also, for those that think or argue that the 904 would require more frequent freshens than a glide, I would have to say I disagree. The parts inside the glide and 904 are both very small, including the clutches and steels. I would have to say they both would require freshening around the same timeframe with this power level you have.
