When all else fails, you can take a ubiquitous Santa Fe steel caboose...not hard to find...and repaint/decal for GTW...
"Santa Fe sold a total of 40 cars to GTW between 1969 and 1971.
They were numbered GTW 75000 - 75033 and 79185 - 79190.
Infomation from Santa Fe Motive Power by Joe McMillan."
Decals for this project are available from K4.
The downside (?) is that this GTW caboose lot would've been long after 4070's heyday. But, if that third rail and the lobster claws don't bother you, ...you know the rest.
Lucas Gudinov