Wonder if I can grow those giant pumpkins up here in Oklahoma in this sandy loam.

Should do great in sandy loam. You'll want to make sure the flowers get well pollinated. It's been a few years, but I watched deal on commercial pumpkin production. Each flower needs a minimum of like 13 visits from bees to get fully pollinated and set good fruit.(that's a lot) and the flowers are only open for a few hours. I would for sure keep an eye on bee activity and if it's low resort to hand pollinating.

Interesting. I just want some of those giant 500lb pumpkins

that would make alot of pies

that reminds me ... a number of years back, our Sunday school class had a big get together around Halloween and had a pumpkin carving contest. All the seeds (likely 20+ pumpkins worth) got dumped into a trash bag and the host of the party asked me if I wanted them. Sure, why not. I had a food plot I had planted a few weeks earlier, so I just spread out the pumpkin seeds along the end and kicked some dirt over them. The following year, I had pumpkin vines all over the place on that end and the deer went bonkers over the blooms. There were actually a few pumpkins that made it thru as well. For the next several years, they would reseed themselves.

FtB, You can buy seeds from giant grown stock, but open your piggy bank. Or try a few more available seed and feed them milk and see what happens. We did this when I was a kid and it works. We used a wick type rope and let the vines pull what they needed, pulled all flowers but 1.

I have Honey bees, bumble bees likely better pollinator for those large petals