That diagram is in the Mopar Performance engine manual, and says it is correct for 64-66, but it is not - at least, it's not correct for a 64-65 (maybe the bracket changed for 66?). When I first swapped my 273 for a 340 back in the seventies, I made a piece according to the dimensions shown in figure 31-25, but it didn't even come close to fitting my 273 bracket. Once you have an actual 64-66 bracket in front of you, it's not hard to figure out what the triangle piece needs to look like, or to make it. As for the suggestion above that you can just run two bolts, I suppose you can. I figure there's a reason the factory specified three, so that's the way I went.