if done properly, I doubt it would affect the value.
From the few first hand accounts I received from people who ran the Muncie three speed, they all suggested that it seemed to be gear perfectly, so I hope you're not looking for a dramatic increase in performance.
Everyone I have spoken to about this transmission cites it's bullet proof durability - and the only reason cited for swapping in a four speed, was for the "cool" factor.
(The guy who I got my '70 Muncie three speed, was quite clear in his assessment of making his car slower - his was a 350-2bbl Lemans of some sort.)
If you do go this route, you can swap over your chrome stick from the three speed shifter into an (OEM/correct) four speed shifter, and all you will be left to contend with is the reverse linkage.
Also, if you do go this route, there may be some people on this board who would be interested in getting that three speed transmission.
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1970 Formula 400
Carousel Red paint on Black standard interior
A no-engine, no-transmission, no-wheel option car.
Quite likely one of few '70 Muncie three speed Formula 400's left.
1991 Grand Am: 14.4 @ 93.7mph (DA corrected) (retired DD, stock appearing)
2009 Cobalt SS: 13.9 @ 103mph (current DD; makes something north of 300hp & 350ft/lbs)
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