wanting to build a 7.21 Firebird

I have been kicking around the idea of converting my 116 weather warrior 7mm rem mag into a 7.21 Lazzoroni Firebird.
I hope to gain some information from those of you with more experience building Savages then I have.
The biggest question I have right now is about bolt head. From what reading I have done this far I "think" I should be bale to use the bolt head from a 300 wsm as there is very little difference in case base diameter (.555 vs .583). Does anyone have any experience with this?
I am pretty sure the 7.21 will fit the magazine from the 7mm rem mag but before I move forward I plan in seating a bullet out further in a 7mm rem mag case to mimic the length of a 7.21 firebird.
If all goes well I hope to order a barrel and start this build in the near future.
Thank you for any help you have to offer.
Cheers
Sean

You'll need to use an ultra mag bolt head in order to get the extra .125 shank length needed by the long action bolt. You'll also need a machine shop to open up the bolt face anyways, so you might as well just use the 7Mag bolt face. At least then you'll know there's no issue with the firing pin length. All you really need is the barrel, and maybe some magazine box tweaking.