ACFT was performing test flight maneuvers when problems with the modified fuel system occurred. Ice blocking a fuel vent line caused a partial collapse of the main (engine feed) fuel cell which produced an erroneous fuel quantity reading. In addition, the main tank overflow shutoff valve was leaking, so tank overflow occurred. The fuel overflow caution light illuminated and auxiliary tank fuel pump feed to main automatically shut down. Due to miscalibration, this system overrode pilot attempts to restart auxiliary fuel pumps. Pilot remained in test area troubleshooting rather than immediate return to base, finally noted main tank gauge continuing to read 'full.' En route to Boeing Field, fuel starvation occurred. Pilot opted to attempt forced landing in small athletic field in residential area rather than ditch in Puget Sound. The aircraft touched down in intended landing area, then bounced across an adjacent street. The arresting action of telephone wires on the vertical fin brought the aircraft to rest in a residential backyard.