Link for 914/6 info
http://www.914world.com/bbs2/index.php?showtopic=74301- it should have a flat /6, 2.0L motor (doh!)
- firewall center engine mount
- no /4 mounts
- factory oil-tank
- different relay board
- "hook" for the oil-line on drivers side rear long
- 911 style ignition switch and column for '70/'71 models
- *no* hole in the front trunk for the /4 air hose to the spare tire
- black "tar" around the front upper shock towers
- different windshield washer bottle
- electric washer pump
- real 5-lug axles
- "914-6" badge
- handthrottle in front of the shifter
- different throttle cable routing, using a tranny mounted pivot
- different paint code on the karman badge
- different VIN, starting with "914" (doh! again)
- no rain-tray under engine lid.
- Dash top has no center vent
some more stuff
Brakes were mentioned as being different. To elaborate further, the /6 has a 19mm MC and the /4's have a 17mm. The /6's have vented front rotors and larger calipers in the front right from the 911T. The /4's are solid rotors and smaller calipers. The rear rotors and calipers on a /6 are larger and unique /6 components. They are not interchangeable with a /4 without a major overhaul.
The e-brake system is shared between the /6 and /4 for the early years.
Visually, don't forget that the steering column on a six is unique to itself. Other than the horn pad everything is different including the 3 speed wiper arm on the right side of the column. The early /4's have a knob on the dash panel.
And we can't forget the gauges. Nothing is shared with any of the /4's.
At the time of introduction, any of the 5-1/2 inch wheels (steel, chrome, Fuchs 14" and Mahle's) supplied by Porsche for a 911 would fit the /6. Remember the best tire size of the day for Porsche was a 185/70.
Also, the transmissions were different. The /6 had 3 different gears. 2nd, 3rd, and 5th.