Mixing different converter parts has been performed for decades. The shop should be able to build converter you need for application on the T-400 trans.
The following does not apply to the T-400 in the OP discussion, just an FYI on the power glide shaft tube comment:
Power glide pump stator tube does not need bored out for 30 spline turbo sized shaft. Often the end of the stator support tube is machined out for a bushing to support the end of the turbo splined shaft.
That bushing is needed when the turbine hub inside the converter does not have a bushing on it. With the bushing in place you need to have a shaft with proper holes in it for fluid flow into the converter. You need a busing support on the turbine hub or the input shaft end at the stator tube.
There are larger shafts for the power glide, that can require a full replacement of the stator tube and pump. Must be careful here for proper matching parts
Dont worry how old you are... it is the mileage on your body that adds up.