After several days and many hours analyzing the task, the engine mount was installed to the firewall. Not an easy task due to post-welding cool down distortion in the engine mount weldment. The weldment hole pattern of full size 3/8" holes did not match the firewall hole pattern of 3/16" undersized holes. The firewall hole pattern matched engineering within 1/16", but the weldment diagonal measurements were 3/16" less than engineering (calculated). The left & right top-to-bottom hole measurements were 5/16" & 1/4" less than engineering (calculated). There was no "best fit" scenario where the weldment full size holes would fully consume all undersized holes in the firewall. Some amount of pre-load would be required to install the weldment without a "snowman" hole condition in 2 of the 4 holes.
With moderate use of two ratcheting bar clamp/spreaders (Harbor Freight or similar), I was able to temporarily bench expand the weldment enough to bring the full size hole pattern into dimensional alignment with the firewall hole pattern. Knowing this, I decided to drill the firewall holes to full size using a deep cutting step-drill to get as far as possible, then finished with a 3/8" twist drill. It took a bit of wrangling/expanding to align and insert the last 3/8" bolt into the engine mount, but once started, a socket wrench was used to "thread" the bolt approximately half way into the smooth bore of the weldment mounting hole, followed by tapping the rest of the way through using a hammer. With all bolts, washers & castle nuts installed, the mount easily pulled up flush to the firewall.