The prim and proper way to mount the engine to concrete is to place one 1/4" steel shim at the high spot to leave room for grout, them shim all the lower ones to same level so the base bears equally. Then set the engine and lightly snug the nuts, then grout under the base and finally tighten the nuts when the grout is dry and hard. This is in most of the older Lister manuals along with how to properly mix the concrete and how much is needed. Lister was pretty thorough about how to mount their products properly. I cheat that process by doing a good job with the concrete and use 1" hardwood boards under the engine instead of grout. Works out well if you keep after the nuts until they quit loosening.