I had put a TKO 600 into my '69 Camaro and LOVED it. I originally bought a TKO (pre 500-600 models) and by the time I got to the point of installing it, the 500-600 series came out. I sold my TKO and bought a 600, mainly due to the higher torque handling and I liked the gear ratio spacing better. It was an awesome tranny and if I had the funds, I would do another one in my truck.
As for what/where, I can't recommend Classic Motorsports Group enough. Bought my 600 from them and they had a ton of great info/tricks. It was still kind of new putting in the Tremecs at the time.
They have a kit for 60-72 Chevy trucks, based on what you already have installed and you would do well to get a full conversion kit from them.
http://www.classicchevy5speed.com/GM...it-Select.aspx
The #1 think about these TKO's they are VERY sensitive to input shaft runout. Make sure you setup your bellhousing within specs or you will cause a lot of problems, like hard shifting, wear and parts failure. Looking at their current kit, they have a new bellhousing setup that is really nice compared to what was available 5 years back.
Good luck and keep us updated.