I saw one used on a '39 Chevy truck once - looked great and made for plenty of room. As far as where to start, if you have the engine out of the way, I'd put the wheelbarrow tub up to the firewall and trace it with a sharpie. Then make a new mark inside of that and start cutting. Go slow and nibble a bit more until the tub fits the way you like.
Good luck, and maybe post some pictures of the final product!
Craig
