Lock-up switch should be your next modification. Depending on how tight your current converter is, you could pick-up a lot of time, as in a full second.
My 95' Junker with a looser than stock converter lost 1.3 seconds and 10 MPH when I lost lock-up.
Then next pass, 17 minutes later so the fluid was still pretty hot, I ran 15 flat @98MPH vs 12.3's, that's almost 3 seconds and 14 MPH.
With a tighter than stock converter, I could still see you picking up 0.5 seconds with a lock-up switch if you lock-up immediately in 2nd gear.
My experience with VP44 trucks is that they run quicker with higher MPH when i would shift around 2700 rpm.
Lastly,
Can you post your time slip or at least list all of the split times. I think with more timeslip related information, we could figure out where your truck is struggling the most, I've got a feeling it's struggling in the 330' to 1000' mark, but post your time slip so we know for sure.