The hardtop trunklids are VERY hard to come by especially in good shape. As stated above there are 3 different trunklids, one for the 2 tailight cars(bel air, biscayne, delray) one for Impala hardtop and one for Impala Convertible. If he is not in a hurry I MAY have an extra for sale, if he is in a hurry there is one on ebay that is listed @ $1500 same guy on facebook has it at $1250. Brent Reese has one 714-720-3438 (Covina, Ca) I have two both need some work but I haven't decided if I want to repair then sell the extra I have or sell it as is and leave it to the next guy. The bottom lip almost always are rough on these. The 348's have some quirks that you want to make sure the builder is familiar with The EARLY 58's used a rope seal and there are 4pieces to the rear main seal plus had the issues with spark plug cooling, later 58 348's had 2pc rear main seal and revised plug location. make sure rear cam bearing is installed correctly with the 3 oil holes lined up at 10-2-6 oclock. I Lamar Walden would probably be the a very good resource for the details that need special attention.
http://lwaengines.com/