Great info there thank you.

The original panel is metal and I am now going to be using a Kevlar

outer panel (wide arch) bonded over it. It will have to have structural regidity also.

also check out the Easy Composites' adhesives @ https://www.easycomposites.co.uk/adhesives

hello Maxidave,

  there are several ways to go about this... all depend on multiple factors of course. 

  *Is the original quarter panel metal\alu  or a composite material (frp\grp) 

  *is this strictly a cosmetic application, or should it need to retain some structural regidity?

  a solution to your substrate issue would be to use 3M's "panel bonding adhesive" part number 08115 http://www.ebay.com/itm/3M-Panel-Bonding-Adhesive-08115-8115-Bond-Automix-200ml-Cartridge-Epoxy-/150741957260?pt=Motors_Automotive_Tools&hash=item2318ebba8c&vxp=mtr   I can assure you this adhesive is nothing short of astonishing.  we've used it to bond carbon\fiberglass to every substrate imagineable (other than a silicon base of course).   

 to apply the grp quarter over the original, simply grind the mating surfaces with a 36grit  or similar paper to give the bonding adhesive something to "bite" onto.  if a metal quarter, grind to clean metal.. if a frp quarter simply rough\key it up to aid in adhesion.

  you are correct in assuming the use of rivets...  if the bonding ashesive stated above is used on all mating surfaces in addition to the use of rivets (which is what I would personally recommend) you should have no issues with the final bodywork cracking do to a poor bond.   I've personally used the 3m brand adhesive to do such additions\modifications to vehicles without ever having a failed bond.

  should you need more information, or a possible step by step...  feel free to ask...  I'll be glad to help if possible.

What is the best way to fit a complete GRP quarter panel over the original?

For the most part it will be held in place with pop rivets but any tips so it will not flex and crack where it is filled around the edge.