Not Ranked
+1 to sticking with standard-length input shaft.
This is a consideration on small-block cars as well, since it's possible to use a standard-length shaft with some bellhousings (e.g. QuickTime 6060) and a shortened shaft with others (e.g. QuickTime 6063). Going with the short shaft and shallow bellhousing allows you to use a slightly longer driveshaft; but of course with an IRS you're not worried about the angular deflections of a short driveshaft like you would be with a solid axle.
I called ERA and talked to Doug when I was considering this, and he recommended standard length on the principle of least surprise - viz. don't put a nonstandard part in your car that may complicate future repair or replacement if the standard part works as well.
Bill
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