Has anyone here used a crank rebuilder that they would recommend?

We use Bill Bune .
www.billbune.com/

Didn't realize that Bill Bune did crankshafts. I just had them bore out my IT400 cylinder. LOL Thanks.

I've contact Bill Bune and will have them rebuild the crank as soon as I can get the parts. Anyone have a lead on where to get a rebuild kit for a 79 IT400/YZ400 (rod, bottom pin and bearing, washers)? I located some on Ebay and hope they don't sell out before the money comes available.

You can always check with Enduronut or DEET

I have personally used Bill Bune to rebuild 2 crankshafts (DT360A and DT1), and am pleased with their work. I was the one to source the parts, I did not ask them to do that step. 
I also used them to do valves and guides on a TX650 head after it stuck an intake valve, and the bike runs well now.

Enduronut used to do cylinder boring but I believe he has retired from doing that (bummer). Just sayin'.

Another source a member here pointed me to is Crankworks in Arizona (I think??) crankworks.com/
They redid a very early '68 DT1 crank for me that needed to have the left seal-side welded and then re-sized after rust worms had eaten the seal surface. They wound up boring the big end pin slightly bigger to accommodate a later style YZ rod assembly as the very early rod I did not seem able to source. Expensive? Maybe, but they saved it and therefore the project.

When I used Crankworks in AZ they asked if they wanted them to supply the parts. I said I had already acquired the oem Yamaha parts. Maybe inquire with them before searching yourself.

That crankworks site is full of interesting stuff.

Indeed.  Time moves on.  All I had to rebuild cranks was a 10 tonne press, a setsquare, feeler gauges, a pair of vernier calipers, a rawhide hammer and a lathe for final checking of runout.

I bet my results would be seen as "agricultural" at best these days.
 

Agricultural or not, it's still how i do mine. Works for me!!.