Hopefully you get the right information and the right builder to handle full-floating bearings. As Ron noted - it is imperative that they are correctly sized to float when they are replaced. Another alternative (depending on the crankshaft size) and if it has dual oil holes in the rod journals - is to switch to later 49-53 rods with tanged insert bearings (like modern engines).
While you may get through this issue, one has to wonder how much he knows about doing a flathead valve job correctly? Does he have the correct pilots to pickup the valve guide bore (not just a OHV pilot thrown into a loose-fitting flathead guide). Does he understand that valve angles and associated valve-to-deck dimensions are different side to side, etc.
Does he know how to correctly balance flathead rods? You don't just grind on the bottoms of the caps like they do with modern rods with 'balance pads' on them.
If he does not know these sorts of things and doesn't have the correct tools - you'd better find another builder or you stand to have issues (not guaranteed, but surely likely). I'm not trying to pick on your builder - but in the end, you probably don't want to be his "first" flathead build.
