She is indeed beautiful and while you want to be safe, her body should be able to accommodate most bodies that come along- especially since you are already in the larger-than-normal category.
My bigger concern is the long-lasting consequence from doing this- if she is doing this because she's into it for her own gain/kinks, or if she's doing it because she feels pressure to satisfy you in a new way- there are lots of great gifts one can receive instead of a potentially relationship-altering one time experience.
Also- you list yourself as 100% straight- this gift is intended for you, watching her with another man? How is your tolerance for dealing with the potential of feelings that might arise if the turn-on you expect is not what it seems? And, I've always thought that a traditional threesome choice for a "100" guy would be two women- not two guys. What if your guest asks you to participate, and he (or your wife) initiates contact in the moment?
I'm not saying that this is a bad idea, or that you shouldn't do it- I'm just suggesting that before you spend time looking for a hook-up that you might want to consider a contingency plan if it doesn't work the way you think.
IF you are in a long term, loving, giving, trusting, committed relationship, and IF we assume that this gift is to satisfy a desire for you to see her with another man, and IF neither she nor you have known strong possessive feelings of the other person, and are ok to have that discovered by a one-time deal, then I'd say proceed.
My suggested guidelines:
Like they say at work, "don't pee in your own pool." Don't look to a close friend or a work associate for your midnight guest- maybe ask a close friend you trust if s/he knows anyone who might fit the bill, so that you don't compromise a currently functioning friendship/relationship.
Consider this an opportunity to explore your own sexuality at the same time. Maybe this tryst can be a chance for you to also test your own tolerance/enjoyment for not just a MFM scene, but maybe a MMF scene. Don't consider bringing someone home unless, in a way, he's a turn-on to you as well. Be selfish- after all it's your birthday gift.
Choose someone who you feel a spark with in a "bromance" way- because you're going to see him hard and highly aroused and/or having sex with your partner. Allow yourself to pick your favorite who you want to see doing that- and it might be easier to find someone open to that scene if he has a bisexual side. Don't be so afraid of labels or what people think to allow this highly sexual scene to be a learning opportunity for you too- and have some naughty, out-of-the-routine, unusual, sexy fun- otherwise, why take the risk???
And, most importantly, if you haven't considered issues of jealousy with your GF and yourself, don't do it. If you have, and you know that this arrangement won't unravel the joy that you currently share, then consider yourself extremely lucky to have this chance- but be aware that it might change you a bit.
Don't rush into anything, be safe, and have fun.