Thanks guys. I was hoping to not have to move the tank...yet. Not in the budget right now. Mainly because someone welded steel over the existing wood bed many years ago. The wood is rotted away, but to pull the bed I have to cut the steel out since it covers all of the bolts holding the bed in. Frankly I'm guessing the only bolts truly holding the bed on are around the edges as the main ones that would go through the wood to the brackets on the frame are sitting loose due to rotted wood...but that is another thread topic.
So back to seats. I was thinking the top of the seat would hit the back of the cab below the glass before hitting the tank. Is that not the case? I was at a car show this weekend. Only one GMC was there representing. A lot of mid 60's Chevy's had updated seats, and still had a tank in the cab. Guess I should have asked their owner's some questions. Maybe they altered the steering as well. For some reason I did not think to check that.