When I put the sender in my 67 I had to get a new float because the solder around the seam was leaking and letting in gas. So try it again I guess.
I found an old thread about replacing the heater core. If you look at some of the trucks on here that have the heater box removed, the only way you can get it out is through the engine compartment. Here is that thread...
http://67-72chevytrucks.com/vboard/s...ad.php?t=69812