Sharps40,
Your research seems correct. I am able to confirm that a V8 Camaro/Firebird T5 will NOT need any mods to the 26 spline input shaft because it is the correct length. However, a V6 Camaro/Firebird T5 has a 14 spline input shaft (IS) which is longer than the V8. I measured my V6 T5 IS and it is the same length as my S10 IS (about 7.25").
The pic below shows the differences in length (V8 T5 next to an S15 T5). Notice also the different lengths of the input shaft bearing retainer.
I would greatly appreciate some high quality photos with measurements of your Astro T5 so I can include them on my website T5 page. I'd like pics showing the length of the IS, IS bearing retainer, shifter location etc. Once it's installed in your truck it will be too late. I will send you my email address in a PM.
Future generations of T5 owners/builders will be eternally indebted to you and your name will go down in T5 history as one of the greatest owners of all time. I promise. 
