Stock 340 fan is the 215 clutch fan, same as 426 Hemi. 7 blade 18” with 2.25” pitch.
If you don’t care about factory correct, just factory specs the GM 772 fan is what I used on my Duster 340. A link to the reproduction fan I purchased and have been running over a year no issue.
Camaro Chevelle 69 70 Nova Impala 772 Engine Fan 7-Blade dated C69 3947772 | eBay