Don I had the same problem with my '46 Panel when I was building it. You really do need 350 springs don't cut them. I thought the 350lb springs were to much. You really didn't say how far along your truck is but what I did with mine was to build kinda of a fake shock with 1/2" threaded rod and a short piece of pipe to take the place of the shock. I compressed the springs down to level the lower control arms(ride height) and left them till the very end of the build. By that time the springs were completly "settled" and I've no problems ever since. I am also running a 350 sbc and TH400 so really go with the 350's you'll be much better off.
