Serious vibs

With a 26'' tire at 80 your in the 4k rpm range. If it doesn't happen in 1st or 2nd gear or free revving in that area, i'd suspect it's a issue in the pumpkin. My buds 70' Cuda did the exact same thing and it was the pinion shaft/gears starting to come apart in his 4.56 geared 8.75.