Just my .02 to the O.P.....you used the term "investment". Unless you really bought well "usually" the best you would do is to get your money back when it comes time to sell. I feel it's best to look at these trucks as a hobby & learning experiance and enjoy them at that. I'm finding out the hard way the the money you "invest" into it will not necessary be realized at the time of sell...and don't even think about recouping labor!
For insurance purposes I'd attempt to insure it for what you have into it, but that may only work if you have collector car insurance and then you are restricted in it's usage.
In short, insure it for as much as you are comfortable with, enjoy it and when you sell it, it will pass on to the next owner in better shape than what you started with.
04 Disco, Gone-Disco died & so did mine
'72 S3 88 - Leakey & Squeaky