Need some A-100 straight axle advise for a 66-valiant???

Look at these articles:

http://www.moparmax.com/tech/i_5-wheel-1.html
http://www.moparmax.com/tech/ii_1-dart-1.html
http://www.moparmax.com/tech/2008/iii_3-rampage-1.html

You will find the work that has to be done to narrow the A100 axle.

Have fun!