I've got one of each. ('79 2440/'78 2640) Owned the 2440 since new, 2640 since '85.
Neither have ever shown any tendency towards overheating.
2440 seems to be the LOUDEST tractor I've ever been around. The 2640 has just enough added low end grunt to make up for what seems to be a lack of same with 2440.
As mentioned, stay after coolant system because of wet sleeve pitting issues.
Neither of mine have hydraulic hi/lo. I prefer it that way. Both have been very reliable tractors.