Machine work isn't getting any cheaper as time goes by - so I would opt to have the block checked and some of the necessary machine work done ahead of time.
You can have the block checked (mag'd), crank bore/Align Bore (if necessary), cam bores also, deck checked and machined if needed and the overbore necessary can be detirmined and done.
You will also now know if the block is worth using and the necesary over-bore size and other spec.s for parts you will be needing to gather prior to the build-up.
When you are ready to assemble the block the shop can Finish Hone each Piston to it's bore for you.
Once you have the block back make sure to completely clean the intire block and all areas including all the oil passages* (if the shop didn't do this for you) with oil galley cleaning brushes & soapy water, blow dry and then pump and spray lube through them and over the block. I use a 'weed sprayer' with storage oil for this - you could also get "marine storage oil" in spray cans from any boat shop and use it.
(* you do not want small pieces of metal shavings to sit around in a block passage and rust in place - they will break lose at the worst possible time...)
It doesn't hurt the block to set in storage at all, as long as it is;
A.) completely cleaned & coated with a good coat of oil or other storage and rust proofing liquid. IMHO - most older blocks I see just sitting around are Not properly coated or stored and are rusting anyway.
B.) Completely covered with a good quality storage "baggie". Summit and others sell these - but, I use some 8mil industrial bags that fit intirely over the engine/assembly and are then tied and taped shut. Dust and dirt are just as bad on a stored engine.
C.) The block is properly supported in an engine storage stand or allowed to sit upright flatly
** on the rear mount flange (where the trans bolts) or flat on it's side pan rails on a flat surface.
(** you may need to remove the dowels or make a flat storage plate from 3/4" plywood that has hole drilled to match the dowel pattern)
store your motor 'hanging' from the rear mount flange on a engine stand for months on end!
Use of one of the many rolling engine storage stands that cradles the engine by the Pan Rail of side motor mount holes is fine.
Hope this helps and others chime-in with their inputs;
ps: Royce has good points and beat me to his post - so I added a few other things that aren't self explainitory to my text - every engine builder has his preferences, you should also check with the shops that you are consider having do the work and get their input on your engine...
Way to go Royce - blow me out of the water...LOL