I've got a 261 in my '51. I used the original temp guage (mechanical/capilary tube type) with an brass bushing adaptor to fit the sensor in the head because the '51 sensor is smaller than the one used in my 261 and other post '54 235s. The gauge works just fine.
Maybe you could get replacment guage for $50 +/- (which will come with the sending unit), a bushing, and move the plug currently in you head on the driver's side near the back to the thermostat housing, where you elcectric sensor is right now.
The Chevy Duty catelog has a picture of the bushing for about $6.00, Part # 24-811. The guage itself is about $50 from Chevy Duty,
Jim Carter has the gauge for $50
http://www.oldchevytrucks.biz/catalog1/0161.html Our Host, American Classics, has it for for $90
http://www.americanclassic.com/asp/...&iyear=1949&ytype=to&exyear=
