What is the biggest engine that fits on a 85 caballero or El Camino?

Answer 1

Go with most any GM small block, you can go bigger of course but that will require some major work. i.e. engine mounts, front suspenstion and transmission with a combination of many other little things

General Engine Block Specifications

While 6 different bore size blocks have been produced, the small block engine can be classified into 3 bore sizes and 3 main bearing sizes. 283 blocks (3.875" bore, 2.3" main), 302/327/350 blocks (4" bore, 2.3" or 2.45" mains), and 400 blocks (4.126", 2.65" mains). In 1968 the main bearing saddles were increased in size from 2.3" to 2.45".

CIDYearBoreMain Cap26219753.6712 bolt ONLY2671979 - 813.5002 bolt ONLY3051976 - 963.7362 bolt ONLY3071968 - 733.8752 bolt ONLY3271968-694.001 (2.3" main)2 bolt3501968-884.001 (2.3" main)2 bolt26519944.001 (2.45" main)2 bolt3021968-694.001 (2.45" main)4 bolt3501968-964.001 (2.45" main)4 bolt4001970-724.126 (2.65" main)4 bolt4001973-804.126 (2.65" main)2 bolt

Notes: 2 bolt ONLY means just that,. otherwise 2 or 4 bolt can mean either design...