Both of the 'cheap' workshops did say that they replace pistons and some other parts as part of the quoted $1500 cost. Though one is a huge workshop which has a high volume of output, so I suspect that they would simply build a block that 'does the job', as opposed to one that stands out.
I'd have to get the shop to check if the block needs boring first, no point buying pistons that are too small or big. Easiest way is to let the shop pick the parts with the best fit.