Some serious wrok on the exhast ports buy building up the shortside radius with brass, some home make 6-3-1 header + 2&1/2" exhaust, some D-Jet mods & you can make some good fuel economy & an extra 1000 rpm of top end. My 1450kg '74 would get 240 miles/tank driven sensibly, 300 miles/tank on the highway doing an average speed of 65mph & about 120/tank at a track weekend. Stock, my 164 woukld do 5500rpm max in top gear with a manual & 3.54 diff. Modied it would do 6500rpm for about 130MPH.

You can mod the D-jet map sensor to work, mine was a roughly repaired unit with a diapram sealed with Silastic & it lasted for years & still doesn't leak.

Biggest conflict for me is this mild 6 (I used a D cam) made the same power as one of my maxium B20's (2x48mm DHLA's, etc) & used less fuel to do it. OTOH, 140's handle better than 164's.