I went out to the warehouse and got the measurements for you on the Curt Trailer Hitch Receiver - Custom Fit - Class III - 2" part # 13430 which has been made for the 1997-2006 Wranglers. To the inside of the upright brackets it measures 42 3/4" To the outside if them it measures 43 3/8".
If that hitch doesn't have the measurements you're looking for on your 1951 Chevrolet Advanced Design I recommend using the Curt Adjustable Width Trailer Hitch Receiver for RVs, 22" to 66" Wide part... view full answer...
The Bristol Double RV Waste Valves # 34466ND14AAR is about 9" long outside to outside. Attached is a photo showing this as well as a review video you can check out. view full answer...