The 54 55 261 will have some differences to the 55 up I think it still used the 216 block design ??? not a100% sure ??? Water pump , distributer gear cam gears / flywheel teeth.... the block casting # are 3703414....3733950 head casting 3703570.... 3836850
For a 216 , look for the small triangular plate near the exh pipe , just above the pan rail --The 235 / 261 does not have that little plate -- FLEET 51
And I believe the 216 had a taller one piece side cover than the 235/261 and does the valve cover have 2 mounting studs in the middle or bolts around the outside ? But I believe the 53-54 235 might have had the 2 stud mount so not a real positive way.
Hey Warren. Have you found a 216 yet? Was reading Sept 4th Old Autos and in section 2 lower right side of page 9b shows Jantzi Auctions. They have listed a 40's 216 Chevy engine. Link to their website is www.jantziauctions.com Auction is this Saturday in Cambridge.
Sorry, I forgot all about this. Was looking for an organization for their parade vehicle. I just came across their phone number today. I will give them a call and see if they are still looking. I know they definitely want a 216 to keep it original.