concerning MPG increase, this is the goal we all look for....your budget will probably determine what steps to take...here are my suggestions(I have had 5 different street Chevy Inline 6's over 25 years-3 250's and 2 292'2 and also now a 292 drag truck).....I have tried MANY different combinations, some,NO!!, All worked because the engine started up and ran, but some combinations worked best-this is the trick finding the best with the components you have added....try these steps 1st-make sure you have a 292 stock intake if you are not adding a Clifford or OFFY aftermarket- the 292 intake manifolds are bigger than the 250 intake manifold-i learned this from MANY roundtrack racers who were restricted to stock intake manifolds for racing and only used the 292 intake manifolds....2nd-update to a better exhaust system-use a STRAIGHT thru muffler...straight thru mufflers help scavange cylinders of exhaust the BEST, without getting as much of the exhaust out of cylinder you will "dilute" incoming fuel mixture and will not have a good fresh intake charge for an efficient combustion...3rd-add headers-this along with a straight thru muffler will help in engine efficency....4th add a better/hotter coil for a hotter/stronger spark-also open gap on spark plugs-try .045....
these above steps are the basic Performance Bolt-On's that will help....if you are willing to go to more intense steps , you will need to add a different cam and have head work done- the head was never designed for much performance but it does work-with better valves (tulip exhaust valves-but will need to have longerr pushrods)and better intake valves.along with various steps to head work you can increase the efficiency of these heads and engines quite alot...with my 3/4 ton 1971 C 20 I get 22+ mpg at 70 mph on Interstate and 10-15 around town(depends on if i drive easy or not)-I have a 4:10 rear gear and use the New Process MY6 833 3 speed with OD trans and have 33" tall rear tires-with the tires and gear and OD -my final drive in OD is Approximately 3.00....I have a highly modified head and a Jacobs Pro Street ignition also-so it took a while to narrow down this combination....with patience and at least a small budget you can start to increase your MPG's....
what are you using for exhaust now?