Fred, there is no difference between a '50 and '51 GMC grille. The same grille was used from '48 on but the '47 and early '48 grilles are three bar and stick out into the gravel pan a lot more. The gravel pan is also different in '47/'48 early GMCs.
$900 for a show level rechrome of a GMC grille is about right in my experience. Chrome work is VERY expensive so get over the sticker shock now. I see Bumper Boyz at many big swaps and they usually have a GMC rechromed grille (4 bar) available for $600-$700 but I don't think that their work is really very high quality. Jez one man's opinion.
If you do decide to have your grille rechromed, it's VERY IMPORTANT that you carefully engrave each piece that you take off and mark it carefully (to survive chrome plating) so that you can keep them in order for reinstall. Each piece is just slightly different and you will go nuts trying to reassemble that grille if you don't carefully mark each bar and upright (left/right) etc.
Just remember: Nothing is as easy as it looks.
