Trans could also be a 741 which is the electronic version of the 740. It does have a allison box(ATEC). Hope that helps. Does sound like it is not shifting into 4th gear. Should happen at about 55 mphJimH
You have a shift pad or shifter tower is the engine is electronic or manual there were a lot of the L-10 E that had the B400 behind it you could have a 741,746,747 ,748 or the B series if a electronic transmission On the drivers side above the transmission oil pan about the middle of the transmission is a tag that will tell you what transmission is there keep your fingers crossed it doesn't say 741 good lucky