348's were expensive to repair when they were new, and time hasn't improved things at all. They have an oddball angle at the top of the block which results in the combustion chamber being in the exhaust-manifold side of the cylinder, with an almost-flat surface on the heads where the valves fit. There is an almost total lack of speed equipment for the 348/409 engine, unless you happen to find new old stock parts somewhere. There was a 3-2BBL intake available as a factory option, and I believe maybe a couple of factory high performance camshafts.
Another option might be a swap to a well worked-over 292 inline six, but that one's going to be pretty spendy compared to a warmed-up 350.
Jerry
