Making video content for 30' x 10' 3:1 Widescreen Looking for some help on how content should be created for an upcoming show with super widescreen blend on it.

The show consists of a 30' x 10' front projection screen with unblended resolution of 3840 x 1080. Video content is currently being created, as the projectors are being blended there is a 600px overlap, the resulting width is 3240px.

Should I get the content made in 3240px or 3840px?

Any help appreciated.
Since the resulting projected image is 3240 px wide, that's the size you should create. Then output two "halves" -- the leftmost 1920 px, then the rightmost 1920 px. There will be 600 px that appear in both parts. That's the overlap.