I'm with Dave...well, not WITH Dave, but behind him...ummm...no, not BEHIND him, ummmm...yeah...:redface:
aww c'mon, you know what I mean!!!
Anyway...I did a similar conversion on my '67, well...OK, it wasn't a 351, but it WAS a 2V to 4V conversion - and I went with an edelbrock performer manifold and a Eddie 4V (650, I think), and couldn't be happier...it was nice to go with the electric choke as an added bonus...and then when I put the MSD billet distributor and ignition in it...wow...GONE were pretty much all the pain-in-the-butt "little things"!!!
Now...if I can just get that carb adjusted back to the way it was when I first got it...MAN it got off the line...until a "carb expert" got to it...now it just kinda "goes", rather than "jumpin".
The 69 "M" code 4V 351W had much higher compression than the 2V did. I believe they used different pistons. By putting a 4V carb and intake on a 2V engine you will improve performance, but not up to the level of the 4V motor. You will need to increase compression (hard to do with today's gas).
Paul G. has it on the spot. The differences between the 351W 2V and 2V were the pistons, the carb and the intake. 2V compression was 9.5:1 while the 4V was 10.7:1. So, to answer your questions. No, the heads are the same, Yes you can put the 4V intake and Autolite 4100 on your 351W 2V without problems. It won't have the same power as a 351 4V but the 4V carb will help. And no nasty converion surprises are hiding out there.
I heard the 4v 351w had different pistons and higher compression ratio. I have also heard that the crank was beefed up to accomodate the extra forces exerted on it by higher CR. Just a rumor I heard.......