Having come (in my dim, dark past) from the world of real 356's in the 60's, let me tell you that putting a rear rack on a fiberglass replica is just not the same.
The steel engine cover (especially on the "B" series) had gussets pressed into the metal to give it adequate strength to hold up both the rack and what was put on it. Even then, the limit was supposed to be a little over 50 pounds - that's it (40 kilos keeps coming to mind, but I can't remember if it's right). I've seen cars with racks really loaded up, but a lot of it might be bulk and not that much weight. Most I would estimate on an overloaded rack was about 100 lbs. distributed evenly over the width of the Leitz (you quickly learned to put light stuff back there, and heavy stuff in the rear "seat" area). And remember, bulky things catch the wind causing a whole 'nuther set of stresses to consider.
There are no strength gussets to be had on the fiberglass engine covers. In fact, there isn't a whole lot of top-load strength in a fiberglass engine cover at all, especially the lower surface where the rack would mount. Once you put the Leitz rack on there with mounting posts in the corners of the grill opening, you have four very small point load locations which, as Wolfgang suggests, could be reinforced above and below the fiberglass sheet beneath the grill such that you'll get a bit more strength by spreading the load out over a larger area. I would use 1/8"-3/16" stainless strap (the thicker the better) on both sides of the fiberglass sheet, bolting through the top strap, then through the fiberglass and through the bottom strap on both sides.
Do all that, and you could "probably" put 30-35 lbs. back there, but I honestly wouldn't do any more than that on 'glass. It simply isn't thick enough back there to provide a lot of strength.
Just my 2 cents....others may disagree, but my take is that a rack on a replica is more-or-less decorative.
On space behind the seats, Chris and I stash a pretty big cooler back there for Carlisle, so, without that I would expect to fit a couple of larger, 4-day, soft bags back there alone, with lots of room for camera bags and snacks (although we eat light...)
gn