Even so, unless Nixon out and out nominates Reagan, I can't see any of the potential VPs besting Reagan for the '76 nomination. Also, while Nixon had other choices (I'm sure EM knows them), the ease of getting Ford confirmed may be compelling.
Nixon won't/can't nominate Reagan, and as
Lord Grattan mentions Nixon's preferred VP, John Connally, wouldn't pass through Congress Watergate or no Watergate
Ford himself, actually, was set to retire because he never thought he'd be Speaker of the House. There are a few outside possibilities, but really Ford was top the of the list for all kinds of reasons.
Nixon has very tight parameters: can't nominate a liberal for fear of losing in 1976 (so no Rockefeller, as Ford found out IOTL), can't nominate a conservative because he wouldn't pass through Congress (no Reagan, no Connally for this as well as his switching parties), might not nominate a Senator up for re-election in 1974, won't nominate a Governor (Nixon didn't care much about domestic policy), and so forth.
Of the narrow options around the well liked Ford, Minority Leader of the House (i.e. not vital), midwest conservative (i.e. fairly acceptable to the base, acceptable to Congress), and so forth tops the list.
Potentials, that I don't know if Nixon considered:
Senator Mark Hatfield of Oregon. Loosely considered in 1968[1] for Nixon's VP, he is probably too liberal.
Senator Howard Baker of Tennessee has to be considered. He's a moderate, insanely popular in the Senate, and from the South.
Senator Robert P. Griffin of Michigan should be considered, but picking him over Ford (as they're both from the same state) would probably be considered a snub.
Senator Robert Taft, Jr, of Ohio. No particular reason besides a reasonable moderate conservative from Ohio, with a famous name.
Senator John Tower of Texas would be considered, but it might be a snub against Connally and he's probably too conservative (despite high profile disagreements with conservatives) to get through Congress.
The crazy choice? Senator Edward Brooke, Massachusetts. He's kinda too liberal and kinda… black. I mean it's nuts, on the face of it, as the Deep South would never vote for a black man in '76 and maybe not the Border South either. But, Nixon was building a long term majority. The South can't stay with the Democratic Party in the long-term and post-Brooke the Republican Party is their only real choice.
Further, black turn-out in the South should help the Congressional elections, and smashing the Northern Black Democratic support is worth quite a lot. So concede the South for one more election, maybe two, screw the Democrats in the urban areas of the North and by the 1980s the Republican Party still gets the South + possibly way better than 9% black support.
At the end of the day if we're not going with Ford I'd suggest Baker, though he'd have to put somebody conservative from the West or North on the '76 ticket.
Would Reagan challenge and win in 1976? Probably, but you never know.
[1] In the sense that a number of candidates were mentioned, but Nixon picked Agnew quite early and passed on the various alternatives mentioned.