New Rebuild Engine Coolant Bubbling

Just had my 289 C -code rebuilt to stock specs.with mild cam.When I drove car home 4 miles from shop, air/gas bubbles were coming out of overflow tank upon getting there.The Radiator upper reservoir was empty and over flow tank was full.Also mechanical temp gauge fluctuating. What really bothers me is the coolant smells like gas/combustion.Rented block test kit for coolant gasses and test came out OK. I would think head gadget breach but my test does not confirm. The builder will make good but it pissed me off.New radiator,water pump,thermostat 180 ,new hoses but builder LEFT OUT lower spring from old lower hose. Any ideas? Are engine block test kits for coolant gasses accurate.Hell when over flow tank sound like a tea kettle it sucks.No problems in coolant dept. before rebuild.Thanks in advance.
 
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Thermostat in back wards ,or reverse turn water pump .Head gaskets installed wrong is the most common .
Thanks for ideas,Checked Thermostat in correct as far as water pump will check with builder.Need to check if front tabs on head gaskets are showing up front. Did install spring in my lower radiator hose today and will test tomorrow. Also pressure tested block at 13psi held other then small sweeper on lower hose at pump.Thanks again.
 
My good friend and coworker Dave built a '70 fastback for himself, left the spring out of the lower hose, and promptly overheated his new 302. What happens is the water pump sucks the lower hose closed when the engine revs. Put the spring back, and his car is all better.
 
Update: So after all the above coolant issues I had another mechanic perform compression test on engine. So had 145-150 psi on all cylinders. But #5 had 130 psi checked out with bore scope,could see coolant dropping down from head into side of upper bore wall. So cracked head? who knows will talk to builder. This I sure wish I would have done a crate 302 this 289 rebuild has been a buzz buster.lol.