another converter question....

I talked to the shop that wants to build my engine/trans today. Im building a 340 and have a 904 trans. On the trans end of it, he wants to install a cheetah manual/auto valve body and a good rebuild kit but he said it would be ok to keep the stock converter in there seeing the 904 converter was smaller and better then a stock 727 converter so no need to change that??.....HUH?? Stock converter on a mild to intermediate 340 rebuild??....Is he correct in his thinking here or will I need a better converter here?? Thanks.