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Question: are the 1/2 ton through 1 ton advance design bellhousing crossmembers (or rear engine mount crossmembers, however you want to term them), the same dimensions?

So, I have limited experience with the lighter duty trucks. I've just had 3800 and 6400 trucks, and I was wondering if the lighter trucks' crossmembers were the same as the 3800.

Reason being, I've got a 3800 frame that I would like to put a '54 or '55.1 crossmember in that has the angled mounts, so I can use the later inline 6/v8 bellhousing.

I just need to know if I need to get one from a 3800 frame or if a 1/2 or 3/4 ton crossmember will work.

Look in the parts manual,

Section 7.229
Picture http://chevy.oldcarmanualproject.com/parts/1929_54/413.htm
Parts listing http://chevy.oldcarmanualproject.com/parts/1929_54/421.htm

The 47-53 list one part number for commercial and utility... I'm not sure but that may be 1/2 ton through 2 ton??
54 has part number for 1/2 and 3/4 ton, and another for 1 ton, and two more part numbers for larger yet.

What does that all mean? Which 54 model crossmember is interchangeable with which earlier models? What's the real difference between the various 54 model crossmembers, are they different shapes/sizes, or only different material thickness(might be functionally interchangeable?), or???

Sheesh, I imagined it would be simpler. I might just have to weld one out of plate

Before doing that find some 54 crossmembers to try and let us know what works.

I'll see what I can drum up

The rear engine mount crossmember is made up of a few pieces.
When test fitting consider you may be able to use the smaller pieces from your original with the main member form the 54. I can't say for sure this is useful, but worth checking.

Okay, I need to see a picture of a 54 or 55.1 crossmember.

I finally got to looking at the link you posted, and it has some great information that's clearing up some questions.

I knew most of the advance design rear crossmembers were 3 piece (1 long that was riveted to the top flange on the frame and two smaller supports that were riveted to the bottom flange of the frame and to the long piece).

The long piece has the same part number for 47-53, and it's the two smaller supports that are different depending on the truck you get (height of frame rail).

The 1/2 and 3/4 ton trucks share the same smaller supports.

The 1 ton and 4100's share the same supports. This makes since because the frame is identical from what I can tell; however, I did stumble on the heavy duty option for the 4100 which basically turned it into a 6100.

All the 4400's, and 2 tons share the same supports.

So you basically had three sizes depending on the frame rail height of your truck.

'54 has the same breakdown: one for 3100 and 3600, one for 3800 and 4100 (except heavy duty 4100), and one for the rest of the heavy trucks.

So, I'm guessing that the 54-55.1 series crossmembers are either one piece, or the parts people at GM started selling the whole unit with the supports and quit assigning individual part numbers.

This brings me back to my first sentence.

A picture of a stock 54 or 55.1 crossmember would solve this for me if anyone has one handy.

http://www.1954advance-design.com/Documents/EngineeringFeatures/slides/orig_Page49.html

http://www.1954advance-design.com/Documents/EngineeringFeatures/slides/orig_Page50.html

Excellent sir, many thanks