Did you first set up your Project as Widescreen? Do you want the Project to be Widescreen, or Standard (4:3), and just use some Widescreen Assets?
That will tell us the best way to proceed.
I didn't set up my project as widescreen. I realize now I should have done that. That said, is there any way to make it widescreen with no letterbox rather than 4:3 with letterbox?
I don't know what Widescreen Assets are.
Unfortunately, PrE does not allow one to make changes to a Project, once it has been setup. You will need to setup the Project to match your source footage, your Assets, 100%. Otherwise, issues will ensue.
If needed, the free utility, G-Spot can be used to determine the attributes of the footage. Frame Size, Frame Rate and PAR is very, very important.
PS - even PrPro has just added the ability to "change horses in mid-stream," because it uses Sequences, which are like mini-Projects within a master Project. Before CS4, even with the Sequences, one had to get the Project Preset correct, before starting. Now, one can just do a new Sequence with new Presets, and Copy/Paste the mis-matched footage into it.