4.250 crank factory 340 block

That's pretty thick in The thrust side, if he checked 12 or 16 spots per cyl you have a nice block. You may have to modify the oil pickup, I would get the billit 2 bolt main caps align home and use arp studs should be fine with no need for a girdle. You can always go 4.125 crank and only bore it .020 for a displacement of 426.