HGNISji
I agree with Harryji and Gyani Jarnailji.
With the religion aspects already discussed and concluded since you have considered collabarating well into the future.
I believe if you are truthful and honest about it all, your parents will accept and understand what is best for you and her.
Don't worry about how traditional they are if you feel that. Most of us like to hold on to the past traditions (sort of ' if it's not broken, then don't fix it' attitude). But we are always prepared for change with the times.
An important point to make clear is that you totally 110% understand their intentions and if they were to disagree- you would understand why.
Parents will consider your opinion and understand that you respect their feelings/wishes and are not being stubborn or doing your own 'marhji'.
You also need to emphasise and reassure them that you do love her, it's not a one off mistake and you have thought and planned well ahead into the future. Along with included them and their feelings in the whole picture.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/love-and-marriage/37947-sardar-dating-hindu-girl.html
Parents have their best interests for your future and need assurance that this decision WILL help fulfill those planned interests.
Most likely at first when told, you know they will NOT celebrate in joy.Instead they will go quiet and silent , maybe say something unpleasant (be prepared for this!!)
They WILL need this time to convince themselves, it won't be long until they see or take notice of marriages gone 'kaput' and unhappy marriages or even false marriages ( where a couple are staying together just for the sake of elders)
Once they have experienced some of these and other ranges of emotions, they will, trust me, begin to see your vision and see through your eyes.
In a nutshell, once you have proven you can see through their eyes and visualise their vision, sooner or not long after they can easily see and understand your vision.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=37947
I hope you go ahead and make the right decision, once you have convinced youself completely (by questioning & reassuring yourself) you will confidently convince them with ease.
All the best and Good luck
Waheguru
Lucky Singh