If you are going to use the stock motor mount position you should try to use the newer late design 455 style, which Pontiac engineers redesigned in 1970. The 59-69 mount position puts all the tension on the middle #3 bulkhead. The redesigned late model 3 bolt mount not only puts tension on the #2 and #3 bulkhead, it also puts an attachment point halfway up the block, and is supplemented with cast iron stiffener ribs to further distribute the tension forces over a wider area.
In this early block with the 59-69 style mounts you can see that only the #3 main bulkhead is subjected to all of the tension load:
In this 70-76 block you can see the tension load is pulling on the #2 as well as the #3 main bulkhead. The #1 and #3 lug counting from the front of the block, plus the raised lug between the 2 soft plugs have extra iron added and put the stress over a much wider area.
In the 69 GP dirt car in my signature pics, I couldn't keep brand new motor mounts together for more than 2-3 weeks. This was when the mounts were actually made from rubber that wasn't the consistency of silly putty. My GP had a 69 428 HO engine, Roughly a 450 HP engine, so it only had the early style mounts available.
I did some study of the stress points, and how I could mount the engine using the stock mount system, (rules dictated that no motor plates could be used). I decided to keep both rubber mounts, and limit the travel with 2 turnbuckles, one on either side of the engine. Because in dirt track racing you run into the turn as hard as possible, then stab the brakes you get some tension from slowing the engine down via the brakes, yeah, different than drag racing.
Looking the engine over I decided that the water crossover would transfer the stresses from either bank better than the rather flimsey aluminum intake manifold, so I used 2 head bolts with the 3/8 stud on the top in both lower front head bolt positions. the lower points on the frame were 2, 3/8 bolts welded to the frame rails.
Getting the attachment points as far away from the crankshaft centerline, also gives the attachment points more leverage over closer points to crank center. Just looking at the mounting points available this looked like the best attachement point in my esimation. Moving the attachment points to the top of the cylinder heads, much reduces the tension stresses upon the area just above the oil pan rails, an area of the block that isn't overly robust.
Anyway this was after a bunch of study on my part, as to why Pontiac engineers completely redesigned their mount system with the developement of the high torque, long stroke 455 engine. They obviously thought placing all the tension on one main bulkhead with the long arm 455 was less than desireable
After 2 years of running that car I retired it, when I pulled the engine the OEM style mounts were like brand new. I've since used the same style system to retain Pontiac engines very successfully in other street, and race cars, it's also a very cheap and simple way to retain the engine with rubber, while removing the twisting stress that is trying to split the block up through the oil galleries that are drilled in the middle of the bulkheads.
Block that used steel mounts on the area above the oil pan rail in a heavy drag car:
