I'm a bit late to this party but.....I was building a 261 and had a similar crack on the front middle main cap. I tried other main caps but they were so far off that line boring would not correct it. So the crack was prepped and TIG welded. Then the bolt holes in the cap were spot faced on all for mains. Assembled with hardened steel washers and stock bolts.
This 261 has Ross 9-1 pistons, some head work , hotter cam and so on. It's still fine after several thousand miles of use....And that means using the performance and not parade driving down the road...
But your experience may differ....
