From the research that I've done to date and my own experience, it would appear that the mid to high 70s would be the ideal temperature to grow giant pumpkins at, in terms of the above ground temperature, if you could keep the temperature constant 24 hours a day. Few people can do that however. The soil temperature you would probably want a little warmer than that so microbes could have high activity to feed the plant. So if you could control the soil temperature, you would probably want soil heating cables making the soil a little warmer.
I have a temperature gauge that records the backyard temperature at 10 minute increments. I downloaded that data and then found the average temperature from July 17th to September 24th. It was all of the data I have available, but it is probably a pretty good average of a season, since it is pretty much the middle of the season to the end.
Pumpkin plants pretty much shut down above 91 degrees and below 54 degrees. So you really need to stay at least a few degrees above and below those temperatures to be growing all the time. It isn't infrequent for me during the summer to get above 91 degrees, although it surprised me that on average I do not go over 91 degrees when looking at the data at specific times of the day. I do however, on average, go below 54 degrees 16% of the time. So my bigger problem is cool temperatures rather than hot temperatures when looking at averages for a specific times of the day.
From 4:20am to 8:05am on average I'm 54 degrees or below. At 5:15am on average I drop down to 53 degrees. I'm looking at ways I can cheaply put heat into a high tunnel using warmed water. I think if I water starting at either 4:20am or 5:00am I can raise the temperature in the high tunnel prior to the sun rising and will be able to keep more sustained growth. From previous data I looked at, 25% of the time I'm outside of the 54 to 91 degree range. If I can add heat in the hoop house in the early morning I can get that number down to 9% and using shade cloth. fans and misters to cool the plants during the heat of the day I think I can get that number down to 2%. The would get me a tremendous amount of additional pumpkin growth.