You need to balance the front & rear diameter , Hotchkiss would match the rear & would be the best choice , or a matching diameter to the Hotchkiss bar
The real issue if you look at any newer car Alum rads are always isolated from the body in rubber mounts as they will not last very long mounted directly to the body & flexing , so brass is better unless you change the mounting for a Alum .
Yes some do crack , easily fixed !
The 302/308 swirl heads flow extremely well . the 308s will easily flow better than X or J castings , the 302s work well also