So I rebuilt my TH350. This was my first time doing an automatic. Is there a way to test my work without having to reinstall? I had zero issues and was surprised how easy it really was to complete. I just would hate to get everything back together and discover an issue.

 

Assuming the end play is correct, and everything air checks ok (you did air check all the clutch packs?) chances are any problems will be valve body related

I use a 3/16 section of brake line on a blow gun to test things before the valve body is installed. Remove the rubber nipple, install the brake line using whatever fittings needed. Then you can bend the brake line to whatever shape needed to apply air to the various holes in the case. You may need to wrap the end of the line with electrical tape to get a decent seal.

 

I did air check everything as I went. All was good.

 

If your able to air test the clutches assembled in the case, and hear them apply with a solid thud (you may get some leakage around the less than perfect seal between the blow gun and case) then any problem will most likely be fixed in the valve body.

 

The only other way to truly test a trans prior to installation is on a trans dyno.

 

I guess kinda the point of my post. Is it a common practice? Recommended?

 

Define "common practice".
For full out race and heavy truck Allisons yes it's normal. Normal guys like us no. Just have to be really careful during assembly.
Large trans suppliers like TCI dyno everything going out the door for obvious reasons.
If you can locate a shop with a trans dyno and the cost is reasonable then why not.