While I like sending biz to this site, I suggest that this is one time to be careful, esp. since, as Soundguy says, this is a spendy changeover. The '65-Sep '68 (roughly) tractors use a different Zenith carb than the Sep '68-'75 tractors. This distinction is missed by 99% of parts sellers, including this site.
The early-late 3000s had this same quirk as the 3-cyl early-late 2000s. I'll give an example of the 3000s. The CNH (factory) web site isn't working well right now so I've dug out the following 3000 info as an example, so check things on the CNH site when it's up and get the 2000 info - before spending $s:
Early ('65-'68) - E1NN9510EA
Late ('68-'75) - E1NN9510BA
To further confuse you, you'll have to be careful on the CNH site because (IIRC), the Holley is listed as the same # except for the last two alpha digits.
Too many sellers and sites, if you look up the part for the early tractor, will link you to the later model carb.
The later model carb is NOT a universal fit. If you trace back thru the CNH parts pages, you'll find (as I did) that the early model governor and linkage is completely different than the later models. What this means, as far as I can tell, is that same Zenith carb body was used across all those years but the linkage (bell cranks, atch points, etc.) differs between early & late.
Besides the OEM factory site - CNH dot com - about the only other seller I ever found who recognized this distinction is farmersserviceinc dot com (note the double-S in the address).
Full disclosure: This subj has two camps of thought on this forum - check thru the archives if you haven't already. There are those who love Holleys and can get 'em to run well and they'll say rebuild 'em. There are others (me included) who went that oft-frustrating route and prefer to scrap the Holley. We bit the bullet 5-6 yrs ago and put the dinero into the Zenith for our '65 3000 and haven't had a lick of trouble since. For me, a tipping point in the decision was when I found that Holleys are NLA from CNH (and haven't been available for some time) - only new Zeniths are available from the OEM.
Hope this helps.