The only 340 block you can do a 4 bolt main to is a T/A block.X block and any race block. The stock 340 block can,t be done. I had both blocks and had no problems either way.I,ve had my bolt main up in the 7000 range many times.It all depends on what you are going to do.My x block was 4 bolt main it seen 8400 all day but I didn,t think it was necessarily the 4 bolt mains holding it to gather.It helped though. MarkBuilding a 422 stroker is it necessary to consider, or even a possibility to convert to 4 bolt main caps on the 340 block? I have seen articles making referrence to this.
The best scenario IMO is new billet 2bolt caps. Once you get into fitting girdles and caps plus normal race type prep the prices get up to where buying a better block makes more sense. Billet 2bolt caps w/studs will be able to live at 600+hp on gasoline. The issue is the main webbing, not cap movement.
And like said above the webbing on the standard 340 block is too thin to allow drilling for the extra bolt holes. It will weaken the block. And you don't need them like said above.