It would help us to know what year, make, model, and engine you're driving. 70A seems kind of low for OE, and replacing with even lower is likely impossible.
Post a pic of your RPO codes from the glovebox. The OE alternator code is one of those and members here can decipher them.
Also post a pic of the current (pun intended) alternator; sometimes the amp rating is etched into the alternator housing.
After some personal research, I see that the 70 Amp figure quoted by mechanic is bogus. I have translated all of my RPO codes in the past. There isn't any specifying the alternator.
The engine is 5.3 flex fuel & Body is LT with these packages
Z82 : TRAILER PROVISIONS, SPECIAL EQUIPMENT, H.0.
1SM : PACKAGE - OPTION 12 OPTION PACKAGE #12
1SZ : PACKAGE OPTION- ?
I don't know what these are, but possibly the alternator spec is part of the towing addons.
Maybe you can determine something from that. Pics of alternators on various parts providers don't seem to indicate any external clues. I'll see if a label is visible.
I'll get a pic if none of this helps.
TNX, Jeff