I'm a trimmer with experience with Marine Canvas and also Custom made Marquees and Tents etc. If it is stitched up with modern "polyester cotton" threads, it will need a good soaking so the cotton content will swell and seal up the stitching holes. Polyester is very strong but needs cotton to seal and pure cotton will rot with age. If there is still a problem, paint some "Bostik" stitch sealant along the seams. Remember that when the top is put up it is in far greater tension than when it is under the sewing machine, so the stitching holes will try and stretch in size. You can make these tops "waterproof but" I can reassure you they would not look trad enough for a hot rod I have HF welded them before but they look shit! I won't comment on pricing here, as we are all guilty of spending stupid amounts of money on rusty old car bodies and motors yet everyone thinks somebody else is a rip-off doing a job they can't do themselves.