Put a set of long 2 piece (high starter) Clifford headers on my 69
Dart today (w/ homemade 4 bl. manifold).
The rear unit rides right up against the steering column.
The column turns and I guess this will be all right.
But the front unit collector is right next to and right under the
front left corner of the 904 trans- in fact I'm wondering if I can drop
the pan w/o loosening the headers. Will I be fighting heat
issues because of this- do I need to install a separate trans
cooler just for this?

I've dropped the pan on my 904 w/clifford headers on the car. Granted this is on a 71 duster w/225 and not a 69, but that shouldn't be relevant to dropping the transmission pan.

I wouldn't worry about a trans cooler. My 904 is happy just the way it is, I've seen exhaust setups on other cars/engines with the cross pipe in close proximity to the trans pan.

If your really worried you can use header tape around the collector in that area, or build a sheet metal heat shield.

This worries me, you don't want anything interfering with your steering.
If the engine torques or the body flexes it could jam the steering. I would bend the pipe to make some clearance.
DD