If you specifically want the early 40 block because of the correct letter in the motor number then there are limited options but if you are just looking for a block in better condition you might consider some of the later production engines. I believe revisions started with the 42 block but definitely the postwar blocks all have the revisions listed below.
The 356 underwent numerous early plumbing, filter cartridge, and canister changes pertaining to the oil filter and lifter gallery oil supply due to an intermittent noisy lifter issue. Later blocks have a cast in port on the block for the final version of the filter plumbing change rather than needing to drill and tap a hole in early blocks (if not done back in the day) for the revised filter return as detailed in the Hydraulic Valve Tappet-- Super Eight article in this service letter.
https://packardinfo.com/xoops/html/downloads/SC/SL-VOL17NO16.pdf A bigger issue is early blocks also had a jiggle pin to control pressure and bleed air from the lifter oil gallery. There were some wear and low oil pressure issues with that arrangement so if you do require and find an early 40 block be sure to check the condition of the pin and hole. Later blocks, I believe starting with the 42 casting, replaced the pin with an actual valve assy as detailed in the Low Oil Pressure article in this service letter.
https://packardinfo.com/xoops/html/downloads/SC/SC-VOL20NO1.pdfThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=249976