The main difference is the heads, and the fact that the 2V has a 2 bbl carb and the 4v has a 4bbl carb, 4V heads bigger but they are not all that great for street use, the 2v heads are more suited for this purpose and with a little machining can be great performers.
Another difference is that some 4v blocks have 4 bolt mains...not really a huge difference in strength tho due the second set of bolts being in a very small area of metal. Anywho thats all i can think of anyone else have something to add?
There were a few differences also. Compression ratio the 2v in 1970 was rated at 9.5 to 1 as the 4v was 11 to 1 plus the 2v was rated at 250hp as to the 4v at 300 hp and the torque for the 2v was 355 at 2600 rpms and the 4v was 380 at 3400 rpms
The short-deck Cleveland (351C) could be had in five different flavors, only three of which were available in Cougars.
70-74 'H' code 2V
70-71 'M' code high compression 4V (not to be confused with the tall deck 351M)
71 'R' code Boss 351 - Mustang only
72 'R' code 351HO - Mustang only
72-74 'Q' code low compression 4V
The 70-72 4V heads had bigger valves - 2.19/1.71 vs the 2V 2.06/1.66 73-74 4Vs had 2V size valves
70-71 4Vs had quenched combustion chambers (higher compression) 72-74 had open chambers. All 2Vs were open chambers except those weird Aussies (no offense, Leon! )
72-74 4Vs had 4-bolt mains, 70-71 4Vs were 2-bolt. All 2Vs are two bolt.
All 4Vs had HUGE intake and exhaust ports. Basically the same size ports as a rectangle-port BBC!!!
72-74 4Vs with automatics were all C-6s, and also got a special high-stall (2700rpm) torque converter and 3.25 rear gears. 70-71s could be had with FMX or C-6 trannies
The Q-code 4Vs had a bigger harmonic balancer (not sure about M-code 4Vs)
The Q-code 4Vs had a very high lift cam
M-code 4Vs were Holley pattern (square bore), Q-code were spreadbore
Actually, the Q-code 4V used dished pistons. Yes, the Boss 351 had popups, but the HO did not.
The Boss heads had a nominal chamber volume of 66.1cc, the M-code 4V had a volume of 62.8, HO had 75.4, 2Vs had 76.2 and Q-code 4V had 78.4. The 351M/400 all had 78.4. Both the Boss and HO had adjustable rocker arms for use with solid lifter camshafts.
For an engine with such a short production run, there sure were a bunch of different parts floating around!!
If you are referring to the HPBooks #36 How To Rebuild Ford V-8 Engines 351C-351M-400-429-460 by Tom Monroe, I can tell you that I have found numerous errors in there, but it is still about the best 335/385-series book I have seen. There is still a lot of information about the engines that isn't in there though -- like 73-74 4Vs having small valves, Q-codes having dished pistons... Those little factoids are things I have found through experience. I just spent $175 for the machine work to have the bigger valves installed in AlphaCat's original 4V heads.