Okay I did a little research to verify my statements regarding weight of the 500 with TH400 versus the GMC 248 with SM 420. Here is what I found.
Cadillac as removed from car is about 670 pounds. Numbers I found varied from about 600 to over 700, but the 670 seemed most credible.
TH400 weighs about 150.
I didn't quickly find a published weight for the GMC 248 that came in this truck, but if I use the Chevy 235 value of 640 pounds I should be under the weight of the physically larger 248, but not by much.
The SM420 weighs about 135 pounds and that does not include the bellhousing, clutch and flywheel.
So while my numbers are incomplete, my conclusion is that I am correct, the total weight is not increasing enough to matter, and could even be decreasing, but again, not by enough to matter.
670 plus 150 is 830 for the Cadillac powertrain.
640 plus 135 is only 775, but does not include the bellhousing and clutch nor account for the difference between a 235 and 248. If those add just 55 pounds the two weigh the same. But even if the Cadillac combo proved to be 50 pounds heavier I'm not worried about suspension being able to handle it.
No suspension worries. But then read my next post...
