Just follow Reckahs PDF for all instructions, it tells you the O-ring part number as well. If you know of a supplier of O-rings you can take the old one there and save some money. A Viton O-ring should be used because of the highish temperature.
The PDF talks about a larger MAF but you can ignore that info if you are not adding them.
For better performance you should add the larger 74mm MAF.
The MAF sensor in the car now is fine, even if the MAF itself is changed to the 74mm one the MAF sensor now in use is put in the 74mm MAF sensor.