Isn’t the Speaker the next in line as VP?
Yes, in the event of multiple successions being needed in a disaster.
In "normal" times Nixon gets to appoint Ford to replace Agnew...
The speaker is next in line should some mischief befall the President and VP. But it it's the President that dies, is removed or resigns, then the VP becomes President, and appoints a VP, who must be approved by a simple majority vote in both Houses of Congress.
That's almost how we got President Ford, the only President not to be elected. The other VPs who became president had been elected VP, Ford was appointed by Nixon after Spiro Agnew resigned and before Nixon himself resigned.