Awesome project! Looks like a handful but amazing things can be done today with the parts available that just weren't really possible before.
The factory a-body 383's did not have the left rear mounting boss. It was a ground down pad over the steering box as you described. I also don't think they used "HP" on the pad either. A D383 would be a 68 model year engine. c=67, d=68, e=69 etc.
Just a thought... You might want to see if there are VIN numbers on the right side of block on pan rail. You may be able to help someone match that block to their car? They are worth good money maybe more so if you can locate to the car it came from.
You can just machine down the MM pad down on a more date "correct" 69 block none would be the wiser except the vin.