the 307 is a 283 block with a 327 crank (3.875" X 3.250") You'll more than likely run into the same problem as I outlined for lamey on this thread as far as counterweight/web interference with a 350 or 400 crank in a 283 block.
http://www.clubhotrod.com/forums/sho...977#post112977
Others on this forum may have better info for you. Want to keep it numbers matching, wrap the 307 in plastic and stow it for later when you sell the car.