There are a few ways around the problem, I probably chose the harder way when I attempted it.
Solutions ive heard to correcting the problem are:
* cut the firewall out and replace with an XD/XE firewall and use an XD/XE heater box, perfect if you wish retain the air cond. (this is how I got around the problem).
* relocate the heater pipes to the air cond holes, obvouisly you will have to lose your air cond.
* reshape existing firewall and plastic weld the heater box, also can have retain air cond.
Thead is here: How to build a v8 XF (the proper way in my opinion)