Turn Up the Heat!

Forming the pieces to your body will be easy in some cases (like the bicep pieces) while it will be a right pain in the @$$ with other pieces. I mean... let's face it... you'll take the back off, put it on to feel the fit, and take it off again quite a few times before you're done... but it is vital that you get the correct shape for the piece BEFORE you use the Bondo on it.
Remember your duct tape dummy? Here's where it gets a moment to shine!
Hang your trouble pieces on the dummy... and heat form them there. If your dummy is anatomically as accurate as possible to your body frame,  you won't need to switch the pieces around quite so much and you can see the process from the outside looking in - a new perspective!






As for WHAT you use to heat form, I used - yes, seriously - a hair dryer. That's all the heat you will need. If you don't have one, you won't need to spend more than $10 on one to get the heat required.
Use your mask, though... heated resin particulates smell just as nasty as wet resin.

With the back piece especially (but the chest as well) you can use the dummy to hang the piece while you heat it.  You'll need to experiment on your own a little fore this part... there's a 'tolerance point' of heat for the resin... up to that point, it's semi-pliable as far as heat goes... but once you reach that point, it becomes a lot easier to work with.
Be careful you don't bend things too far (this is why we have multiple coats both outside and inside the pieces)!

For larger pieces, duct tape the snot out of it, then heat form it directly on your dummy... your dummy is reasonably close to your body shape, so even if you move a section too much, it is still much more recoverable than flipping the pieces on and off your body... and a lot faster!

An extra hint for the back piece? Duct tape it down as you are heating it; come in over the next day or two and give it a quick once-over with the hair dryer while you're working on other pieces... bu the end of it the back piece should be contoured much more closely to your own back's shape.

As for the resin, it will 're-dry' as it cools and the new shape will be more-or-less kept... and now you can get a better idea as to how everything is supposed to fit together!

"Oh THAT'S where that goes!"