I had a couple of one foot long 2x4s in my garage. I just put them under the very back of the tank, stacked on top of each other. Worked great. It helps if there isn't much gas in the tank. Mine was full unfortunately.
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If you are doing the Power Commander V install I would remove the tank. Much easier to get to the last 3 connections that way. For the Fuel Moto (Area P) propping up the tank is fine. I just stack up two pieces of wood I had lying around on top of each other at the rear of the tank (closest to the battery).