Fox-
The 289 HI-PO or K motor came with a 5 bolt bell housing until 1966 and then with 6 as does the 302. The 289 HI-PO motor has a stronger casting down in the webs and larger thicker main caps than regular 289's and 302's. The Boss 302 is basically the HI-PO 289 with a .30 higher deck and 4 bolt main bearing caps on the middle 3 journals. It had basically the same strong webbing, but it came with threaded Freeze plugs and 1 less water jacket above the cylinders.
I ran a Boss Block as a 289 in my GT-350 R back in 1970 in one race before we retired the car from SCCA Competition. I still have the engine, unassembled of coarse and was going to put in an FIA Shelby or Kirkham before I changed and went with a 427. If your interested and want more info E-Mail me----
brucerobles@earthlink.net By the way you can get 400+hp with a 289.