70 is going to be very close to what you need.
What's more important it the size of the old intake. If it was small, you aren't going to need a bigger jet.
I just put a 15mm bing on with a KN filter on a stock Magnum with the E50. It runs with an Estoril. The 15mm comes with an 82 jet....and I'm running rich.
I would guess the 14mm with a 70 is going to be very close. Obviously, everyone's running conditions are going to be different. Do what everyone else has done, and do a plug chop. Go buy 4 or 5 jets from 68-74 and test them. That's really the only way.
I'm in the same boat right now.......