New owner of a 84 turbo diesel pickup - I'd appreciate any advice from forum members. Truck has engine trouble and will probably need a rebuild or a replacement. How much better an engine is the 4d56 than the 4d55? Or, put another way, is a 4d55 worth the cost of rebuilding or replacing? Another alternative: are there any other engines out there that might fit easily into this truck? Thanks for any thoughts....