Replacing a tube:
make sure you push the valve stem in as far as it goes once the tire is mounted. This makes sure the part of the tube near the valve isn't trapped under the tire bead. Then you can pull it back out far enough to be able to push on the tire pump head. use the threaded nut that travels down the outside of the stem to hold it. Don't make that too tight, though.
Tire pressure
Try 105-110 psi for your weight of 245 pounds.