The best place to put the vents is just after the projectile reaches maximum velocity in the barrel. If you put them before, you'll lose performance, if you put them after then the deceleration wont be optimized. This point will of course vary according to weight of projectile, firing pressure etc.Currently I have a 40" barrel. How far from the base should I drill the holes to vent the barrel into the sleeve? Once I get it together I will experiment to see if I need to extend the barrel and the sleeve.
You can see it in this simulation at about 14 inches in a 40 inch barrel, using a much smaller chamber:
Note however that the chamber is much smaller than the on you're using. You really should aim for a small chamber at high pressure as opposed to a large chamber at low pressure, because the latter will not give you good enough acceleration.
For the above example I halved the chamber volume and doubled the pressure, technically the same amount of air - but we increased velocity by 12%:
I really recommend plugging numbers into GGDT (That I don't think we than D_Hall enough for!) before starting construction, it will save a lot of trial and error and wasted PVC.