I tried the same thing: distributor boot from a Mustang 5.0 I used to own with the same results, while my GT never had an issue. I think the distributor positioning of the Ford 302 vs. Chevy 350 is the key element (front vs. rear).
Wire loom, sealed at the ends with slicone, and a good industrial o-ring at the distributor base seems to work best. For me, anyway...