Chevrolet: 1955 Chevy bottom end noise - possible engine swap, speed bellhousing, crank pulley

Question
Hi, I have a stock 1955 Chevrolet 3100 with 235.5 cu. in. six and four speed trans. Lately I have experienced a slight knock from the bottom end - sounds like main bearings. Also, vibration at low speeds and there is play in the crank pulley when you move the fan by hand (with engine off of course and trans. in gear). Could this be the harmonic balancer? I'm considering replacing 235 with 1970 Chevy 250 inline six or GM 273 small block. Will either engine bolt up to my existing bell housing so I can keep my four speed?  Thanks!

Answer
Dave ;
Hi -  The play that is in and out is not that bad , it is the play from left to right or up and down. lol.
but it must have some miles on it if the crank can do that. I would go with a small v-8 like a 283  or a 327-350. but if you want to keep the six thats cool. I think that the 1970 six should fit with no trouble.and the 4 speed  bellhousing should fit most all v-8s  etc. in or around that year. Do you still have the front motor mounts ? you might have to use other mounts with newer v-8s but if you go to (jc whitney.com ) they carry all of that stuff-tell them I sent you.   OK >>>