Newer models in the 40 series i'm sure would have had a copper gasket . Slotted head bolts are from the 50's and 60's and will only fit with certain cylinder heads.The newer ones have the later hex head bolts (easier to remove as well) and quite different around the bolt holes of the head (not recessed as with the "cheese head" screws)
mix and match with the parts and you'll find a lot of things will fit together nicely. Preparation is the key.
Worth mentioning that on the 40 models fitted with the villiers carb the carb size is normally 1/2".
I dont think there will be much (if any) difference between the 2 carbs you have except 1 might have the 25;1 needle jet already fitted. (perhaps the choke is hinged on the left with one and on the right with the other) Unless of course you have the smaller 7/16" version which will still fit but the motor will run a bit erratic (unlikely but worth mentioning)
pics we like pics

jon