it's cheaper/easier to source another block/crank if you do buy the rods and pistons and bearings first then the machine shop tell you they are outside tolerances i suppose. but as above, if you go for 74.5mm pistons and give the spec sheet to the machine shop you know it will have been machined properly to spec... same with oversize bearings. I've been thinking about building a forged engine for a while but i'm sure people like taggy and turbo grant will be able to advise properly