I'd do it, if you or your shop are prepared for the work it'll take to make it straight and fit right. You seem to already know that it will be inevitable that it won't fit correctly right out the box.
I eventually plan on replacing my OEM front bumper with a fiberglass once I get the future paintjob. The fiberglass ones do take a ton of fitting, bodywork, and massaging to get them to have a factory fit, but you have the benefit of them never warping or losing their shape. I can't stand seeing '79's with the notorious floor-jack "grille warp" (usually on the driver side), which even my '79 has. With fiberglass, that's not an issue and they'll look straight and new forever.
