Worth the effort?

Not knowing what you currently have for a convertor...
Any convertor's stall speed will vary depending on engine's torque output, a car's weight, and the design and manufacturing of it. If an engine makes more or less power than the convertor manufacturer anticipated, or in terms of the lessor expensive units the car is lighter or heavier than the manufacturer's "average", the stall speeds will vary. Generally, the cheaper the convertor, the more likely it will be a little off from the advertised stall. So I would say don't feel too bad, and if you really want to see if the convertor's the culprit, look to the health and tune of the engine. Because if the engine's down on power or not what was anticipated, the convertor stall will be lower.
Now, as for opinions, I'd rather have a close-to-stock 727 behind a mild 340 in a street car. There are valid opinions about going faster, lighter, lower first gear, etc. But unless the 904 is getting gone through and upgraded to the 727 type high gear clutch drum, my opinion is to stay with the 727. If this was a race car and you planned to drop some coin on the swap, I would go 904 for the reasons I mentioned. But for what you have, I think the 727 is the best way to fly. I'd also make sure the engine is holding up it's end of the deal too.