I assume when you had the 2/4 servo pin out, the servo piston, servo case surface, and seal ring were serviceable. The grooves on the servo pin are length IDs. There is a procedure using a special tool and a torque wrench for determining the required length. The length takes the place of band adjustment screws and lock nuts seen in other transmissions. On old school transmissions you could adjust the band according to the book and then deviate a little depending on the output of the engine. You might blindly go with the next longer pin but make an attempt to see if the band is dragging on the reverse shell.
The 2-3 flare is an indication that the 2/4 band is releasing a little too quick as the pressure rises on the 3/4 apply piston. If the 2/4 serve is at the limit of its travel, it would probably spring back quickly when the pressure starts to drops. Another indication that not everything is right in the 2/4 band apply department.
Also consider that if the servo piston is leaking that the resultant pressure drop may have affected the clamping ability of the 3/4 piston causing the 3/4 clutch pack to slip and burn previously. The initial failure probably occurred while pushing the transmission in overdrive. So do an air check on the 2/4 band servo to see it there is a leak somewhere.
Yes if 3rd gear is holding the slip rings are probably good.
Good luck on this. You might go back to the old 2/4 band and extant servo pin which were probably well matched. I bet you are getting tired of taking this transmission in/ out/in/out/in of the truck.