Are pacesetters really that bad? Do they last? I've read of the occasional fanagling the joining of the downpipe. I believe your 88 has an egr--the 91 does not. You could probably use a newer header(seems most go with apn/knockoffs these days) and block it off. . .
If emissions are not an issue, then you could get away with runnign the newer header how it comes. IF you need your functioning EGR, then you would need to cut and weld a piece for the egr.