Yes. My # 2 & 3 main webs cracked all the way to the cam bearings. This was N/A on Nitrous with some detonation. Any V6 production block will fail in the main webs if you detonate it enough. Most builders use billet caps at least on the # 2 & 3 mains. The caps have to be fitted to the registers and the mains line bored and honed. I went with an RJC girdle after that failure, and a better tune with a progressive controller. No problems since. The girdle involves machine work on the block and caps, and line honing. The other issue is the head bolt holes cracking into the water passages. Countersinking the bolt holes like a 109 block helps that problem. Some 4.1 blocks have a lot of core shift and thin cylinder walls. Sonic testing is not a bad idea. Also, final bore size is dependant on the rings available for that bore size. Custom pistons can be made any size, but rings may not be available for that bore diameter. Follow Mike's build in this thread if you have a good machinist and you will be OK, or find a vendor on here that knows how to build one.