the FTBR kit really served me well the last 10 years along with other mods! but that is exploded now! its getting A moser SRA & coil overs &strange double adjustable shocks,wolfe racing control arms....and so on!!
Your car bogged way too much on that pass, the sweet spot for my car on the 28" (1.44 100% stock suspension, 1.36 with the Viking DA non coil over setup all around) was 3800-4000, any higher it would break parts. The low 3000's bogged bad and couldn't get out of the 1.5's, so I didn't waste much time.
I think IF the rear could stay together, the IRS is a very capable 60'er.