well the title is a lie. i have a 91 firebird and just did a 305 to 350 swap and for some reason my cooling fan will not come on unless i turn on the a/c. and i am still using the 305 computer and wiring harness. engine runs fine. and also are there 2 thermostats? one for the computer and one for the guage? cause the guage does not work either.

 

Single fan?
Was TBI or TPI?

I'm going to assume 305 TBI...

Did you install the fan switch from the pass head of the 305 to the 350? If not - that is you issue with the fan - the fan switch is in pass head between #6 and #8 cylinder, with a 1 wire connector. When the temp gets hot enough, the switch activates the ground for the fan relay and activates the fan.

If there is no provision for this on the 350, or it's plugged on the 350 and you can't or don't want to remove the plug, may I recommend a seperate fan controller from Flex-a-lite - Summit sells them and they are great! Has a probe that inserts between the radiator fins to sense temps, is fully adjustable for on temp and off temp, has provisions for manual on switch in the interior, and mine is 3 years old and works everytime perfectly.

There is another coolant temp sensor located on the driver side head, between #1 and #3 cylinder (on the 305) that will also have a single wire connector - this is the one that sends signal to the guage. Locate this on the 305, and see if you can't swap it to same spot on the 350 - that will solve your guage issue.