Round nose 310? Serial nr 6078140 is 1957 model year.
TXS 635 carb venturi is MS p/n 46-457, Case p/n B51815. Not sure where available.
Case had full kit p/n G11050 six years ago and may still be available. Basic & immediate carb kits also available at Napa, Carquest and some other auto parts stores.