Chuck,
How's my panel worked out? Some places to start
http://www.g503.com/forums
There is a radio forum, and a very large collection of people who play with vintage military radios. The expertise there is frightening and sometimes deeper than some of the expertise here. Robert, wa5cab I believe, is a very extensive expert on aircraft radios. I would start there first.
http://www.qth.net
There is a military radio email list there. I haven't used it, but I know some of the heavies from G503 frequent there.
FYI, my 1944 BT-13 44-89867 was equipped with ARC-5 radios. Apparently, the late war trainers were all equipped with the series, which were also not known at that time as the ARC-5- that is a later designation. The ones in my aircraft were mounted in front of the front spar on three trays at an angle. I wonder if the T-6 may have had them mounted on the wing center section in a similar fashion? The power supply is the big thing on those- I don't think there is a convenient place to put it on the fuselage sides on a T-6.
Good luck, and send photos.