Unfortunatly due to the filters here at work I can't view the photo, but I can probably help from memory. Just a assumption form your description.
The original modulator's can was approximately 3 1/2 inches long, with the vacuum nipple at 90 degrees to the end, on the side of the cannister. Also cannister diameter is about 1/2 size of original.
The replacement is probably 2" long or less, with the nipple on the end in the center of the cannister.
It will work, but IMO it will give a softer shift feel than the original one.
That smaller type was originally used on the TH350 trasnmission, the larger type was used on the TH400. You probably will not know the difference unless you thrash on it hard, the upshifts will not be as crisp.
The last Chevy "hot rod" I had was a El Camino, 77 m/y that was upgraded from the 305 to a 350, 327/350 cam, headers, Edelbrock intake, and a B&M shift kit. While in Michigan at the low altitude here, it regularly chirped the tires going into second. When I moved to the southwest, at 4600 feet above sea level, the shift got "mushy". I fixed that by changing to a TH400 modulator.
While I don't have the correct GM parts catalog for the pre '75 Cadillacs, but using the truck catalog it appears GM dropped the large can modulators, superceeding them to the smaller TH350 style. I seem to remember this from a few years back this same question came up at work.
I am working on acquiring a pre '75 catalog, if successful I should have one in a couple of weeks.
Some aftermarket source should have the large can available if this becomes a issue. I may be able to locate one too if needed.
P.S., saw your Lifeliner at Flint last Saturday. I introduced myself to Paul as we have communicated online and on the phone in the past.