I'm currently knee deep into restoring my 69 Firebird. The car originally had a 4.1 L OHC 6 in it, but about 20 years ago the previous owner put in a 350 SBC. When ripping the front end off, I noticed that the guy had pounded the crap out of the firewall to allow room for the distibutor. Obviously the engine is sitting further back than the original. I want to convert the car to a 400 Pontiac block. Is the cross member the issue, or is it as simple as changing the engine mounts?  

If the Chevy motor is sitting too far back, likely they used the wrong mounting holes in the front crossmember or the wrong combination of mounts. I doubt the trans crossmember needs to be changed, but they may have moved it back as well, check for extra mounting holes in the subframe, you should be able to see if the holes were ever used. To install your 400, first you need to determine if the block has the correct mounting holes drilled and tapped for the Gen I Bird. If you have a 67-69 block they should be OK, or if it is a 70-75 block it will have 5 mounting hole but all may not be drilled and tapped (you can handle this if you are careful and work slowly), if you have a 76-79 400 you will need adapter kits available from most Pontiac parts suppliers. Use the original style frame and block brackets and trans mount, relocate the trans crossmember as needed, and that leaves the driveshaft length as the only variable (it may have been shortened to accomodate the extra setback of the Chevy installation. Keep in mind that the TH400 trans is longer and uses alarger diameter output shaft yoke than the TH350, ST300 or manual trannys and may need a shorter driveshaft.Try to check out a similar car at a show or cruise and see how everything fits. Good luck.
Steve