How rare is it?

[ES ES 396]

I have a 1970 Chevelle with a 307 engine and four-speed transmission. My brother bought it in 1981 for $200 when I was 14 years old. If I remember correctly he told me the 307 was out of a '72 Nova. I always assumed it was converted from an automatic to the stick shift. He quit driving it around 1989 and left it sitting in the garage. I tried for years to get him to either fix it up or sell it to me, but it just sat. (It was pretty rough when he bought it and was just getting worse.) Sadly, my brother passed away in 2008. My sister-in-law, who swore the car hates her, let me have it. In the paperwork she gave me with it I found the protect-o-plate and found out this car came from the factory equipped with the 307/4-speed drivetrain. I've tried to find production numbers for this but I can't. I would think it is pretty rare. I was planning to put a big block in it (NOT an SS clone though!) but I'm thinking if it's really rare it should be restored. Any info or opinions would be appreciated.