Put what you want into your car. Funny how die hard Ford guys want a Ford in a Ford. Does not mater that this is a 51 Merc which had its own Flathead Merc, not Ford.
Now for the real decision. The front sump on the Ford motor presents a challenge when oil pan meets the cross member. You have a couple of choices there: 1. Use a 66-77 289/302 oil pan (if they will fit your 351) or make your own. 2. Modify the cross member (you still have to do a little of that with 1 above, or set the motor much higher than you should)
Centerline, if you drove your rods the way I do mine, availability of parts any where in the USA on any day of the year is important. I certainly would hate to sit in Timbuktu for 5 days waiting on a part to be shipped in to get me going again. That, plus the ease of installation of the SBC in the older Fords, vice the 2.5 inch longer SBF, means a lot to me. The SBC "Cookie Cutter" became just that because of the millions made, the glut of after market parts, interchangability over 50 years of production, and just plain old reliability. Many of us have seen so many ingenious engine swaps and have nothing but great words for this ingenuity.
Trees