As far as economics go, my (admittedly limited) understanding is that the Chicago School was still in its infancy around this time and that supply-side economics only emerged as a direct response to the stagflation of the '70s. So my guess is that Reagan would govern like a conventional conservative Republican of this time period, rather than jump on board the supply-side economic train 12 years early. In fact if Reagan's then-still-brief record as Governor of California is anything to go by, it seems he might actually not be averse to raising taxes if he thinks it's necessary - not just sales and vice (alcohol & cigarettes) but also on the highest income bracket.
Aside from that, IIRC Reagan's main complaint was that the US should've just declared war on North Vietnam and gone all-out against them instead of, essentially, pussyfooting around with the Gulf of Tonkin Resolution and putting boots on the ground (covertly or overtly) everywhere
except NV. So I'd definitely assume an escalation is in order with an early President Reagan. A larger and completely un-hidden Cambodian Campaign, mining of North Vietnamese harbors, and earlier & constant
Linebacker bombing offensives are the minimum I'd expect - at maximum, Reagan might actually try to secure a congressional DoW against the North Vietnamese with the public promise of ending the Vietnam War swiftly in one last big all-or-nothing gamble.
In other matters of foreign policy, Reagan was a staunch backer of Taiwan and opposed any sort of reconciliation with the PRC. So at the least I'd imagine no detente with China as long as he's POTUS.
Domestically Reagan was huge on law & order around this time, as demonstrated by his forceful breakup of the People's Park protest in Berkeley (followed by deploying National Guard forces to Berkeley for two weeks) and consistent support for capital punishment. I doubt he'd be remotely sympathetic to or inclined to compromise with urban rioters in the slightest. Police militarization was beginning to take off in the late '60s - the LAPD SWAT was founded in 1967 and had their first real baptism of fire in 1969 with a successful siege of the Black Panther HQ in LA - so perhaps President Reagan could massively accelerate & expand that trend? I also doubt Reagan would actively roll back desegregation (he's not a Dixiecrat and didn't do anything to enforce segregation as Governor AFAIK), though
apparently he was a strong opponent of busing. As far as more general social conservatism goes, perhaps Reagan could lobby for a School Prayer Amendment as he did in 1982 - it's rather early for the culture war over abortion (which Reagan partly legalized as Governor although he came to regret that decision, same as no-fault divorce), and definitely way too early for the one over gay marriage.
And, of course, I think it'd be safe to guess that he'd have no problem encouraging governors to send in the NG (and personally backing them up with paratroopers) to smash especially large and troublesome riots. Groups like the Weather Underground would probably be even more violent against, and in turn be treated mercilessly by, the Reagan administration (which could use their antics to justify harsher crackdowns and militarization of law enforcement) - something like the Days of Rage could easily result in that group's leaders being shot dead rather than arrested, for example. At worst America's cities might spiral into a mini-Years of Lead, although in the face of escalating violence and 'urban guerrillas', suburbia and the working class might very well come to back Reagan's law & order to the hilt (real-life examples of this phenomenon include initial support for the NG in the Kent State shootings and the Hard Hat Riot of 1970, where blue-collar workmen in NYC clobbered college-educated activists protesting 'Nam & the arrests of Black Panthers).
In general I don't think Reagan would try to be nearly as sneaky as Nixon: his 1968 self seems too blunt, too inexperienced & too prone to wearing his heart on his sleeve for that sort of skullduggery. If he is to commit any impeachable offense it'd probably be in pursuing the Vietnam War too zealously or cracking down way too harshly somewhere domestically, not for dishonesty. That said, given his charisma (Reagan being the one to talk about the moon landings should make for a truly legendary speech!) and lack of an odious reputation compared to Nixon, I think he might have better odds of surviving impeachment proceedings as long as he can keep some conservative Democrats & Rockefeller Republicans on-board. If he needs to appease the liberal wing of the GOP, maybe he could even attempt the domestic equivalent of Nixon going to China and name
Massachusetts Senator Edward Brooke, a black man, his running mate in 1972.
All in all - the more I think about it (and I've been doing that quite a bit today haha), it sure seems that an early President Reagan would make the '70s an even wilder time, to put it mildly!