No oral is a must as she will be able to tell the size/smell difference.
To help with the size/shape difference of his cock, you can regularly blindfold your wife and then fuck her with a variety of strap on dildos including the ones with realistic skin and have a bulb for squirting cum into her. You could even have a friend cum for you during the day to her photos and then put his cum in and squirt it into her. After she is well used to having a variety of sizes of realistic feeling dildos in her, 3 months to a year depending on how often you have sex, then you can sneak a guy in who wears the dildo attachment strap with his own cock poking through and let him fuck her. She will still feel the strap and think that you are still using a dildo on her. When he cums inside her, she will think that you are just squeezing the bulb again. If you want to fuck her afterwards and/or clean her up afterwards, then you need to do that during your strap on dildo practice sessions so that she is also used to having that also done to her. This way everything that you are planning to do to her, you have already done to her on a regular basis and she is already used to it and will feel safe with what is being done to her.
Also position her so that her ass is at the end of the bed, so that you/he can fuck her without getting on the bed, either with her legs up/spread or in the doggie position. Have her hands tied to the upper corners of the bed, so that she can't remove the blindfold easily. Yes the hood would be better, but be prepared that she may prefer the blindfold. The other way is to work her up to the hood during the 3 months to a year of getting her used to having sex this way.
Once you do it with another guy once, make sure to go back to the realistic dildos. Then bring in other guys on a regular basis, just not every time. This way she is always getting a variety of dildo's and cock's so that they will all blur into being just being a variety of realistic dildo's in her mind.
If she is not on birth control, then be very careful of her fertile week to only use the dildo's as her getting pregnant by a guy who does not look like you, she will notice after the baby is born and then you will be in big trouble with her. If the guy looks like you, then that is better, but it is a big risk to your relationship, maybe not right now, but it could be in the future when something unexpected happens.
