Another Pacing question

I have a gravel race coming up. It is 125km with just shy of 4000m of climbing. Start and finish in same spot. I can’t get a GPX file of the course prior, the course will be fairly loose gravel. Finish times will be about 6hrs ±30mins.

If this was a road race I would happily aim for a IF of around 75.

Given the descents will be very steep and I won’t be peddling and the climbs will also be steep.

How am I best pacing for the long climbs?
My FTP is currently about 300…

Kind regards