A 351C into a 67-70 Falcon or Fairlane is a fairly straight forward swap. The 351C has large square heads that would seem to cause problems but they actually assist in the swap. The exhaust ports on these heads point down rather than out like on the other small blocks. I think the 351C headers are much easier to install than regular 302 headers. The 351C has a shorter block deck height than a 351W (9.208 vs 9.503) which helps with shock tower clearance. Specifics are listed here:

Engine assembly:
The engine sits down on the stock 289 or 302 motor mounts. The 351C has the same 6 bolt bellhousing pattern as the later 289/302/351W so it will bolt to a C4 or smallblock C6. The C4 is preferred because it absorbs less horsepower and weighs less. 157 tooth or 164 tooth flywhees can be used. Be sure to use the appropriate block plate for the transmission bellhousing. NOTE: A small bell C4 with a 157 tooth flywheel requires a small starter (gear reduction, etc). The large stock starter will not fit with those parts. The weight difference between a 302 and a 351C is about 75~100 lbs. This will not really impact ride height . Stock Falcon springs are around 260 in/lbs, so that's technically .38" The preferred headers are from Tubular Automotive, but I have been told that Hooker makes some swap headers (probably Super Comps) for the Fairlane that should work. The Tubular headers are the best quality, but will need to have one inch removed from tube #3 on the passenger side to pull the pipe away from the firewall. Both 2V and 4V headers fit. I little trimming is required on the frame tail that supports the trans mount where the header pipes pass by. The 67-70 Falcon and the 67-70 Fairlane share engine compartment dimensions. The stock 351C pan clears the crossmember fine and the Milodon 9 quart 30927 pan also fits, but rolling back the front lip of the crossmember is recommended to allow for drain plug removal (it's on the back of the Moroso pan).

Hood clearance:
With careful selection of intake (a low rise performer) and air cleaner (drop base 2"or 3" element) there are no hood clearance problems. Note: A drop base on a low rise manifold will NOT clear a DuraSpark cap. These recommendations are conservative, so experimentation may yield different results. I run a Weiand Xcelerator with 1 3/4" of spacer, Holley 4150, drop base air cleaner with 4" element. This combo penetrates the hood by about 1". YMMV.