I test fit the cab on the chassis with the new powertrain setup and I learned a few things. It was nice to see it resemble and actual truck again, even if only for 30 minutes.
It turned out that the firewall and the valve covers on the big block were just kissing. There was almost nil for clearance at the distributor too.
I know some other members here have gotten away with running a big block in the stock position. I was hoping that would work for me, but alas I'm not overly surprised that it didn't pan out. Based on this test fit I knew I needed to move the whole powertrain assembly forward by an inch or so.
Then I poked around the inside of the cab and the hump where the shifters come through. The SM465 is about 1.25" longer from its front face to centerline of its shifter. So I decided to move the motor forward 1.5".
I didn't want to move it too far forward. When I had the big block in my 2wd 64, I moved it the full 3" forward and it really seemed to shift the trucks weight too much over the front wheels. I also didn't want to lose all the space between the water pump pulley and the radiator like on my 64.
Anyway, I didn't get anymore photos, but I pulled the cab off and powertrain out. Then I removed the stock engine mount brackets, as well as the bellhousing crossmember. I then installed the 67-72 K-series engine crossmember and clamped it in the target location, with the motor mounts exactly 1.5" forward of the stock position. I put the 73+ transmission crossmember about where it belonged. Then I put everything back on one more time to make sure it would all fit okay, and wouldn't require any frame trimming. Everything fit fine so now I have to clean and paint the new crossmembers, drill the frame and then I can put the motor, trans and transfer case in their final resting places once and for all. Whew!
The real kicker here is that I would not have been able to so easily move the powertrain forward if i hadn't gone to the married SM465-NP205 setup from the divorced SM420-T-221 setup. And the reason is that I wouldn't have been able to just shift the motor mount brackets and bellhousing crossmembers forward. They are both mounted to angled parts of the frame and would have required extensive rework and fabrication to make work, or just completely custom crossmembers. So, in the end, everything would have ended up the same either way.