What are my best options for a motor swap/upgrade on my 03 maxima 3.5, 6 speed manual?

Hello, not a great forum to ask about modifications. You'll be best to find a local shop near you that customizes in customer motor work. A high end machine shop. You can try some local repair shops near you using this link: www.repairpal.com/repair-shops ...and perhaps they'll have a lead. Good luck.