Thomas,

No idea what plane you are building, but Cozy Girls had some plates made from aluminum and alodined. http://www.cozygirrrl.com/aircraftparts.htm

With threading, generally depth should be 1.5 times the bolt diameter -- for a steel bushing.  So 1/4" bolt, means a 3/8" steel bushing is fine. I like to go a little thicker with aluminum (you mentioned 28tpi which implies steel, as for aluminum I would use 20 tpi).  1/4" steel plate seems almost too heavy.  0.071" 4130 plate with a welded in bushing would be fine.  Primer and painted and no rust worries (My fuselage is 4130 tubing).

Tom Walter