I hope no one minds my opinion here but I am retired from the International Sheet Metal Workers Union after 35 years and I know a little bit about this. Aluminum is milled in different series, 3000 series, 4000 series 5000 series and 6000 series and more but these are the most popular. the lower the series the softer and easier it is to form. The most used series in shop fabrication use has been type 3003 which is very easy to bend or form without worrying about it cracking. You can have different degrees of success all the way up to 5000 series when forming aluminum. 5051 is what we normally consider marine aluminum (and some say aircraft) and it will form fine depending on how sharp a brake(bend) you need to make. If you take 6061 and try to brake it it's probably going to split into two pieces. Maybe twice in my 35 years did I ever heat a piece of 6061 to make a bend. If the 6061 is available to you or even free I can understand using it if it needs forming but if you have to go out and buy it I highly suggest using a lower series aluminum. If I ever decided to armor plate my jeep I would use a 5000 series material, it's still strong enough to take the abuse and yet soft enough to form it. I am in no way questioning anyone's method, just trying to add a little to the topic.
Jbeemer, I admire your ability and by looks of your shop I also believe you know what you're doing.