frame identification

[dmacs66gmc]

Hi all!
I have been lurking on this site for a while and find it a very useful source of information.
I have a 66 gmc long stepper that I am restoring. Through the process I have come to the conclusion that the frame and rear end are not for a 66.
When I order parts for a 66 I am getting the wrong parts more often than not.
First thing I found is that the cross sills under the bed floor are thinner than a 66 should be. Also found differences in the emergency brake assembly. Seems like the front end is from the correct era as I have not had any issues when ordering parts. The pre bent brake line kit I just bought looks like it is for routing of the front to rear line on the drivers side. My frame has the break line running down the passengers side.
There does not seem to be a serial #on the frame just what I believe to be 2 part #'s on either frame rail. I have included pics of the #'s on the frame. Have also attached pics of the frame and rear end.
Thanks in advance to anyone that could help sort this out.