If the shop will overbore/hone w/out the slugs then I'd question them. Some shops do it - but most shops will want the pistons to be sent w/ the block just to be sure. Good luck man. STAY ON THE PHONE W/ MAINSTREAM OR BG - and take your time. The race is AFTER you get the build, not before.

Also - depending if you are gonna boost it or not, be very careful as you machine and re-machine a stock bore block. For boost your final bore may be over what's "safe". 1st get the bore checked to see where you are on teh worst sleeve (if they are hand bored, trust me they are not all equal). Find the biggest overbored sleeve - get that number - then make sure there is enough material there to overbore for consistency throughout the remaining sleeve. Work from the BIGGEST bore backwards to get a worst case scenario - and figure out your target overbore. This is just the measuring phase - MAKE NO CUTS> With that knowledge in hand, then make sure that your piston manufacturer has an off the shelf piston to support what overbore you will end up with. Then supply the slugs w/ the block when you get it machined. They will likely mark which slug was machined for which cylinder w/ a sharpie. Then drink beer.