A/C- non A/C conversion??

Yeah the mounting holes are completely different, the ac hole is twice the size the motor holes actually vary a little from year to year for both ac and non ac. Non ac boxes mount with a j hook into the fresh air hole in the cowl plus with studs coming out from the blower motor area. Depending on the year there can also be a brace welded to the bottom of the cowl. Some years also have a small plate that mounts over the heater hoses too.

If your going to do the swap look for an early style box that has the metal trim ring that mounts on the engine side of the firewall. To me this is a nicer look and mounts a little better plus if your holes are a little off then the trim ring will cover that.

Its really not that hard, i welded all the wholes shut and then just held up the empty box inside the car to mark my new holes. It was tempting after I welded everything just to skip it all together and have a smooth firewall but my welding isn't perfect and I didn't want to spend the rest of my life doing body work in that area. Plus I knew the first time i got stuck in the rain I would wish i had a blower for defrost.

Look at this thread http://www.bigblockdart.com/forum/showthread.php?2092-How-To-for-Heaterbox-Motor-Modification

I used this thread to also put in a smaller motor so if i go big block down the road i'm not going to run into and issues with the motor being out to far. You can recess the motor and also there is a model of motor that is a lot smaller. With both of those mods the motor is almost flush with the trim ring.