The 12 bolts are going fairly high in price due to their being original equipment on the rarer 1st generation Camaros. You will be paying extra for the collectibility factor. And a 10 bolt can easily be built to withstand the abuse just about anyone can dish out on a 69 Firebird convertible.
If you are dead set on a 12 bolt then you will find it's a very straightforward swap. Since they were original equipment on the Camaros, all the parts are available for the swap. I believe that your original driveshaft will have to be shortened but the conversion u-joint is available... You may be lucky enough to find a Camaro driveshaft that would bolt right up.