The ripples in the metal filter allow the fluid to get to the intake hole, while the filter hugs the bottom of the pan, which also helps keep the pipe connected. GM had an earlier design 64-67, if I remember correctly, that occasionally would dislodge, so the most common filter setup made from 68 until 1990, is the redesign.
If I remember correctly the last chevy 1 ton trucks that used the TH400 were a little bit deeper than the standard passenger cars, 1990 was the last year before the 4L80E. You may be able to use one of their tubes if you only need 1/2 inch more.
The other avenue would be to buy an aftermarket steel tube extension that would likely be too long, and shorten it to the length you need and weld it back together. Hope something I posted helps you out.
__________________
Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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