I would say yes for a 40lb yak, are you sure the yak is only 40lbs? Sit in's are usually a few pounds lighter than SOT's, but 40 lbs still sounds "light" for a 9' yak imo. Suggest you veryify the weight, but the OEM crossbars should still hold more with issue.
I would fish out of the yak a few times before "rigging" it out so you can see/determine what you "need" vs what you "what". Still need a good comfortable PFD no matter what, a good paddle, and maybe a rod leash, the usual emergency stuff, and then start surfing the yak fishing forums for tips and ideas. Enjoy.