Man, your shop is giving you some bad info. No doubt that you should be shooting the TH 400. There is nothing wrong with the shaft. They just gave you the wrong size.

Do you have any expectations of shooting 70# one of these days? If so then the 350's would probably work then, but not now. I'd take them back and see if they will swap for 400's.

What kind of vanes are on them? If they are Blazers then they might work with broadheads, but if just standard vanes then 2" isn't going to give enough steerage for broadheads.

Just some advice, as it seems you aren't getting it now, but next time you want to make a purchase come here for some advice before you buy---not after.