TH400 was dscontinued in F-bodies after '74 because it wouldn't "fit" with the catalyst installed. TH350 (M38) was the trans installed in '76 T/As no matter what engine they came with. If the trans pan is a rectangle with one corner "lopped", it's TH350. If it sorta looks like "Texas", it's TH400 (M40 in a Pontiac).
We use XE262H for the level of engine you describe. XE274H needs at least a 2,400 RPM "stall" and works better with 2,800.
3,000 RPM @ 70 usually means 3.55 gears. If a "big 10 bolt", 3.42s.