what headers will fit on my 91 integra 4-2-1

Megan Racing, DC, T1R, Mugen, HKS and probably a few others. You'll have to notch the cross member.
For the 4-2-1 set ups he wont have to notch it. Only set ups like the 4-1 ITR and the Tri Y can be a problem with the fitment. And it's only certain tri y headers at that.
 
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