My first truck is a 1949 GMC pick up, and I need ideas

I got my truck for $1700, it's in decent shape, but it has no interior, what would you suggest i put in it for seats? I am not putting it back to original. I'm also not sure of what it's sitting on right now, my dad thinks it might be a S-10 frame, if anyone can tell me where, I can get pictures of any specific place that will help ID what it is. I need to know so i can put a gas tank in it. If you wan to know any specifics that might help, please let me know, you'll just be helping me out. I appreciate any and all help i get.  

48 and 49"s (some of them) mostly had the gas tank in the rear under the Passenger side of the bed.  the fill port was just in front of the rocker space panel.  49 (some)thru 55 1st had the tanks in the cab and the fill port would be behind the passenger door.
There are many under bed kits for the tank.

As for the chassis, are there any cuts or welds that would make you think chassis swap.  S10's are too long and you have to cut the horns off the front and rear of the chassis, then make a bracket to mount bumpers back on.

Most mid size seats will fit right in and the same for the chassis on the full framed midsize cars (IE: older monties, riv's, etc.)  In the seventies these were being mounted on those chassis because S10's were't here yet  
07 GMC crew diesel
53 3100 original
53 3100 rat
51 3100 streetrod
49 F1 panel

The bed has been shortened, and the gas tank was in the the cab, they plugged up the hole, so we plan on just getting one for in the bed, does it matter what the frame is? Or do they sell some kits that will fit any year? Thanks a bunch!  

You don't need to change over to a new chassis if you have the original.  There are many kits and upgrades for the original.  If you want to change tho you can buy rolling chassis complete for your truck.  The cheapest way to change and in my opinion the easiest is to throw the body onto an S10 chassis for a few reasons
* they are cheap and abundent
* they are a true truck chassis
* they can be altered to fit with minimal welding
* you can get many many kits to adjust ride height
* you can keep the original drive train and run the four banger if you want (although that does leave a big hole under the hood)
* you don't have to change the original body to fit the chassis (you can always go back to the original chassis)
* you can have a modern ride for litteraly pennies (ok dollars)  
07 GMC crew diesel
53 3100 original
53 3100 rat
51 3100 streetrod
49 F1 panel

Oh and don't forget if your break down on the road you can get the parts anywhere.  try buying parts at Joe's auto in Bummf*** Michigan on a Friday for your 51  
07 GMC crew diesel
53 3100 original
53 3100 rat
51 3100 streetrod
49 F1 panel

k thanks again for your help