340 replacement block.. with a roller cam?

that's correct, for certain lifter types. Different manufacturers have different tie bars, and there are some that do clear. I built a 416 recently for a fellow who insisted on using a factory block, just because he (or his previous builder) ground thru his R block clearancing the tie bars. You really need to reaswearch the designs, and dont talk to salesman...they have said they clear for a while now.