How about a sturdy enough crossmember from left to right to catch the front of the radio, all floating below the roof skin.

Another perhaps better option is use a different radio with just a remote display which is easy to mount, put the rest of it under the dash or seat.

Or here's the radio option I'll be using, no real/bulky radio at all just an amp, volume knob, ipod cable, and ipod of which many have a radio function.
http://www.jalopyjournal.com/forum/showthread.php?t=243821
Using these parts:
PAC IC-AV A/V Output Cable for IPOD Video
PAC LC1 Remote Amplifier Level Controller
And an amp

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-