You're talking about the squared cover toward the RF passenger area? It's the battery cover on the 47-55.1 trucks. It's made by Dynacorn and can likely be found at your favorite vintage truck parts supplier:
http://www.dreamershotrods.com/Batte...yandGMCPU.aspx
If you mount the battery under the floor spend some time to plan shielding from exhaust heat.
There are round plastic covers used in the marine, camping, and bus worlds that might work for the master cylinder:
Amazon.com : SEAFLO 8" Boat Round Deck...