i got a a 351 windsor for free and want to swap it in for my 302 how hard will this be.

 

Straight bolt in. Youll need to get your exhaust "Y" pipe cut and widened about 3 inches though. Other than that, the 302 and 351W are the same engines basically. I did this to a 78 E-150 Econoline van I had years ago. I even used the 302 distributor and exhaust manifolds and trans flexplate.

 

You didn't say the Year....
The 5.0 Roller cam engines use a different distributor gear from a flat tappet cam.
Steel vs Cast iron, and the two should not be switched, will ruin a camshaft over time.

What year 351W?
Anything after 1976 model is worthless till you get to the Lightening engines.
They used the same heads as 302 and those starve the 351w for air flow.
I got a 75 model that even with a 2 barrel will burn the tires off the pickup, I pulled it from a Granada.
I had another from a 77 T-Bird. IT was a dog and was not anymore powerful than a 302 would have been.

To each their own, if you want a 351W and want more power find the correct heads or go aftermarket and start from there. If your going to leave it stock might as well have a 302.