It will fit in the chevy with a few mods. Such as depending on the tranny in your 53 chevy you may have to change it. You will have to make some motor mount mods and you will need some shorty block hugging headers or some to go through the fender wells.
The merc would need a few more mods, such as a full GM drive train. But anything is possible with time, effort, and or allot of money
Thanks, hope I helped.
yes it would the only problem you,ll run across in one of those old cars is how to modify the engine mounts, the rest is easy because of the ample amount of room they had in them,good luck.