I used to weigh 340 pounds. I've had three R3Ts. With the first one, I thought the stock seat would be pretty good, but it would get tiring after about 150 miles. I bought the long haul seat, and thought that would be good, but it would get tiring after about 200 miles. Then I sent the stock seat off to Russell and had it custom made for my weight and geometry.
The Russell seat took a little while to get used to, but in the end, it was a great seat. It would hold up for full day rides, just as Russell's name (Russell Day Long) implies.
When I traded that first R3T, I sold the Russell seat. Less than a year later I missed the R3T and replaced it. I went right to ebay, bought a stock seat, and sent it to Russell for modifications. Everything was great.
I recently traded that R3T for my current one, but I kept the seat. It's great.
I now weigh in under 200 pounds. The stock seat works very well for me, and I think that maybe even the bike feels a little sportier with the stock seat. But on a Day Long ride, even that Russell that was custom built to support my 340 pound As*, still holds up well.
Don't waste money on the Long Haul seat. It's not likely to be a solution. If you're having trouble with the stock seat, the prescription should be a Russell custom build.