You might check into the price of dies...BEFORE...you decide which way to jump. I think Ed has the best price...at least the last time I checked...$125...but that may have changed. Otherwise you probably will get sticker shock and/or have to wait a LONG time for them to be made. AND...lets not forget the price of the reamer, unless you know a gunsmith that happens to have one, then you STILL have to pay HIS price.
50 Martin dies run about $110 and 450 NE dies about $156 plus shipping from CH4D...and who knows how long...I've been waiting since Jan 1, 2010 for my 510 MAKATAK dies from CH4D, at $232.57, and don't expect them for another month or more.
I thought about the 500 Martin also for the converstion but it didn't have enough hair so I put it aside...and there are several other ways to get you more "stuff" out of the 500 S&W NEF...there are several .500 cal rimmed AND rimless cartridges that would make excellent conversions...the 50 B&M Long at 93 gr H2O, and 500 MDM at 114 gr H2O both using the RUM case, 50-140 at 132 gr H2O necked to .500" and Ed's 499HE at 152 gr H20.
Going with Ed for all your supplies and chamber work for this conversion and in this case will get you going much faster and without all the hassle and/or heartache you can get by whizzing around gathering up all the pieces and parts yourself. Besides even at 6 bucks a case it's chumpchange considering just how many times you can reload each.
Take it from me...IBINDOONDIS wildcatting stuff for a whole long time, I do most all my own lathe/mill work, chambering, rifle building, die making(sometimes), bullet making, pieces and parts making...the whole shmear...and I garoontee...I will let ED do the chambering work for any cartridges I happen to like that he has developed...it is MUCH cheaper in the long run, he knows what he's doing and he doesn't take cartridge development lightly, and he knows more about the NEF and what it can and can't do than a whole lot of wannabees...like me.
Anyway...just some things more to think about
Luck