Probably the most acurate way to get the toe-in spot on,would be to measure from the insides of the front wheel felloes(where the spokes fit into,or your disc wheel felloes),this would give the most accurate toe-in reading,as you wouldn't have to worry about rims being out of true or tyres being out of true as well.
Once the toe-in is set(1/8"),then you can try to get your rims to run as true as it's possible to make truck rims run.I've been through the same hassles with my '28 truck,and everything is just fine now.The maximum castor angle unloaded should be no more than 2 - 2 1/2 degrees.
As for balancing the front wheels,if you can find a good front-end man/tyre shop,probably one that deals/works with trucks,you may be able to get the wheels balanced on your truck.My front wheels were done this way,and now there is no more shimmy or excessive vibration from the front end,even running along at 50-55 MPH.
:)
