Just to make sure I understand: you have a 1911 carbine kit you plan to use for attachment of a sling, but do not plan to add an actual buttstock (which would make the 1911 a NFA regulated SBR). Correct?
Use an AR pistol buffer tube. It's a simple tube with no attachment point for a buttstock.
I'd personally not use a regular buffer tube with an attachment point for a stock; that might get one into trouble. But there's perfectly legal non-NFA AR pistols with short barrels utilizing a pistol type buffer tube.
Something along the lines of this.