The sending units with return niples use 1/4" return... even if the supply nipple is 1/2". Since you will get supply from the sump, you could use the main supply nipple on the sending unit as return... buy whatever size sending unit you want up to 1/2".

1/4" return is pretty restrictive and may make it hard or impossible to get fuel pressure where you want it for the carb depending on pump output. I used the 1/4" return nipple for a while with EFI and an A1000 pump. With that pump, the lowest pressure I could achieve was around 30 PSI... OK in my application since I was shooting for 45 but not so good for a carb.