Re: Xc To Xa
The XC master cylinder won't bolt to the XA booster, you have to change the booster and master as an assembly- no big deal. Everything else will fit. Depending on what radiator is in the XC you may have trouble bolting in the lower mounts to the XA, but it can be done, again, no big deal. The diff will go straight in. The handbrake cables won't. You have to use a combination of the XA and XC stuff to make it work, but if you have two cars to pick parts from it should be OK. The speedo cable will be on the other side.
Now, the real question is, where are you going to find a 351 4 speed XC GXL....?
You would be better off selling your XA to me and buying another car. Is the XA a factory 302 with original engine at the moment?