I also have a 78 mini race car, it was my first car and I have replaced almost everything on it. I have often though about doing a FE project on another mini but with our winters here and the rust problem I`ve stayed away. However if I had the cash I would by a fibreglass mini sprint pick up and build a hybrid. A series motor up front, 850cc and a plug in electric in back.
To start if you are going to change anything about the engine it's a good idea to read "tuning the A series engine" by david vizard, it's mostly for race but there is fuel economy tuning info in there too. A carbed mini should be able to do 50mpg UK at 70-80km/hr without any fancy aero mods or engine changes.
First thing I would recomend is to pull off the 12s and put back on the 10s. This will lower the unsprung weight and will use less gas to accelerate. Also if the 10s are 145 they will be more aerodynamic. The 12s do have a slightly taller gearing which would help if you do a lot of 50mph+ driving. Check out this site to see the RMPvsSpeed
Guessworks - Gearbox & Speedo Ratio Calculator You can probably sell or trade the revs and buy new 145s if you need them or if your 10s have 165s.
If you never use your back seat you can save a lot of weight by removing the seat covers and the carpet from that area. There`s also a lot of sound deadening material that in my opinion never did anything to make the car any more comfortable. I would recommend pulling up the front carpet taking all that out and putting just the front carpet back in.
If you are willing to spend money and spend some time playing around I would say invest in a megajolt. This will eliminate your distributer, points and coils and replace them with an updated electronic ignition from ford. You can buy all the parts for around $200 USD, the mini forum will have a lot of into on it and for a mini specific sensor see
A Series Crank Trigger Kit - Detailed item view - Specialist Components Online Shop I have a lot of their parts, including this sensor on my car, I just haven't got around to wiring up the mega jolt yet.
One part I would avoid is any kind of cheap LCB header or exahust system. After reading a lot I pulled my large diameter system off and replaced with a maniflow medium bore LCB header and an RC 40 side exit exahust. It's a lot quieter, but test after test has shown that the mini 5 port engine responds very well to this exahust and will develop more low end grunt with it.
I would be very careful of doing a grill block because of the side mounted radiator. However that means that basically the non radiator side of the grill is useless for cooler. Minis have been known to benefit from ducting air from the grill to the radiator. All this hot air then needs to escape through the side of the car, usually through the wheel well so allowing it a quick easy exit will mean you can block off more of the front grill and accomplish the same level of cooling. Also the fan is reversed from the optium direction as it pushes air instead of pulling it. A front mount radiator with an electric fan will improve FE.
Try building a plastic air dam to stop the air from going under the car, but also be aware the fins on the transmission are made to cool the oil so you would want to add an oil cooler to compensate.
If you have an SU you should also be able to play around with different needles for pretty cheap and lean out the mixture. The SU is a great carb, I just bought one this winter after years with the webber 45.