Does anyone knows if someone builds a mandrel bent dual exhaust system for these trucks? Mine is a 64 short wheel base with a 4 speed and 292 with headers.

 

A couple problems, #1 being that the Y block just isn't popular enough. #2 is probably that there is no standard for headers so it is impossible to make a system that fits every header.
There is nothing that will be pre made, off the shelf.
You can usually find a shop with a mandrel bender locally. Be prepared to spend much more than you would at the average exhaust shop.
You can also get a universal kit to cut and weld up yourself.
Another option, a mandrel bent system does look nice and will have an edge over the crimp type but in most cases, it just isn't necessary so why not go to an average exhaust shop and get something that will function just fine.