Check with Vintage Speedster or JPS for a wiring harness. They may be less than a West Coast Metic item. The cost depends on who you have do the work. I had a buddy do the work and got a great deal. Ask around I would imagine the day would take an experienced person at least one full maybe two days. There should be easy access. No interior Lights, speedo, oil temp,..etc. Mark everything you take off and even some digital pics would help.
Check CB performance for the cluster or gauges. You may have to do some fiberglass work on the dash to get them to fit. If your IM is like mine was, I had 914 units(bigger) origianlly and wanted the green face units. Kirk or John may have some in stock too (VS & JPS)In fact go to their websites and they have a list of items they sell to build your car.
Don't cut corners. I would invest in quality items that you will enjoy using and that will be a selling point if you ever sell. And if you want to sell let me know, I would love an IM with a tube frame. I am running on a pan now.