Reply to "Building a Gr4 replica"

ZF strength will be the least of your problems unless you decide to contest one of the Vintage endurance racing series in Europe. On most public roads, you simply cannot use that much power for more than 5 or so seconds, so the combination will have a much longer life than in competition. More pressing may be the flexing you'll experience from the rear of the chassis. Seam-welding the whole car from the firewall back plus some sort of chassis stiffening system will help. As you probably already know, the stock upper bay-brace is useless, as are the aluminum aftermarket bars. You'll have fun sorting out what you need in shocks, anti-sway bar sizes and tires/brakes. FWIW, with a slightly too-big cam and aluminum heads at 10-!/2:1 or slightly higher, you can probably use the highest possible pump gas without needing an octane-additive. Keep us posted on your progress in taming the beast!