OK ... Whose bright idea was it to choose a BBC anyway? :(
I finally got back to doing a little work on my project.
The cab was 'sort of' set back into place on the truck, but I had to remove all of the pickup box to frame bolts and slide the entire bed back a couple of inches in order to drop the cab into it's general area.
I could see that those tall valve covers weren't going to fit into the existing firewall, but clearance for the HEI dist looks OK.
So I made some measurements (using a square against the frame) to make sure that the cab was centered. I also tried to take into account the assembled height with the cab on it's rubber mounts.
Next I took a paint marker and drew some rough guides for the required clearance. I wasn't quite sure how to proceed ...
Should I remove the cab and bash in the firewall, or
cut into the firewall to remove the obstruction and box it in later?
Well, I had a close look at the driver's side from inside the cab and realized that the bashing option was probably out because of the reinforcent panel for the steering column and brakes.
Also, being on my own with no available help to lift the cab off and on ... I went with the cut and box it method.