Virtuemart Core Change AND Extension Creation

I already have a core change that adds Volume (CBM) to Virtuemart. The last VM upgrade overwrote the modified files. The project is two-fold: FOR NOW: I have a change log with code for the change. I entered the code as instructed, but a few of the line numbers are now different due to modified files. I have most of the functions working - but for volume in cart and orders. I need you to review the change log and figure out what needs to change and make those changes. I need this done asap as this is a live site. FOR THE FUTURE: The core change is not ideal precisely because of the risk of overwriting the code. I would like an extension (or plugin?) that does the same as the core change - but remains safe and functional after upgrades. This is the scope: 1. Adds a Volume field to Administrator/Virtuemart/Product/Product Dimensions and Weight 2. Adds a Volume field to Administrator/Virtuemart/Product/Custom Fields - Takes that value and displays it in Product Details - Takes that value and displays it in Cart - Takes that value and displays it in Invoice - Takes that value and displays it in Administrator/Virtuemart/Orders - Takes that value and totals it with all Volume values in Volume Total in Cart - Takes that value and totals it with all Volume values in Volume Total in Invoice Layout and fields exist in cart, product details, and emails.