Re: H4350 vs h4350 EXT... Different data /powder?
Personally, I'd begin with the Sierra starting load and work up.
The Sierra manual max is actually @ 54.1gr of H4350; they were using Federal cases and Fed 210 primers for 2,880fps out of a 26" Savage barrel.
I don't know what Hodgdon used for cases and primers or what kind/length barrel they used to get 2,938fps. Maybe they'll tell you.
It's a little interesting to note that for the Sierra bullet they expressed pressure in the older CUP and the Hornady bullet in PSI. Sierra doesn't state pressures.
There's also a similar difference when comparing the same Sierra bullet but using IMR-4350. Sierra's max is 56.0gr while Hodgdon's is a compressed load of 60.0gr.