It's not the easiest of guitars to do a mock build on. Don't fit anything permanently to start with. just wrap tape around bridge posts so they stay in the holes, don't fit the inserts yet.

I'd also wait a week or so before doing too much fitting. When I got my ES-3, it took that long for the wood to settle down and the neck to fit in the pocket. remember it's come from 40°C heat via a jet to sit in an unheated PO warehouse with who knows what in the way of temperature and humidity changes on the way.