Question about my non original slant six

I have a 1975 dodge dart with a slant 6. Yesterday I checked the cast numbers and they came up as a 1968-70 cast slant 6. Now my question is since in 1975 the slant 6 went down 90 hp and the 68-70 the slant was rated 145 hp is there anything different between the blocks or is it all smog equipment that drug the hp down? Im stoked about the 68-70 block. Even though im saving up for a v8 lol. The slant has a external fuel pump that makes it easy to start up and it runs pretty dang smooth. Thank you

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