Peioa,

No stock rear apron was standard or a GM accessory/option. However, I think that Patrick's has made a rear apron that looks "period correct" (not listed on his website; but, if you are interested, you might want to give him a call.

Here are some other rear aprons:

RSA47 - American Classic Truck Parts

66-081 - Classic Parts of America

SM322 - Jim Carter

Tim