Early Abody Doug’s 450 header install BEFORE engine going in

If the headers are the under-chassis type, I would certainly try it. I have used this method many times on the 67 up A's, always with good success. But, I install removable studs, in the end holes, of each head to hang the headers on, and hold them in alignment, while I struggle with the bolts. And I cut the gasket so that I can drop it between the header and the head AFTER the header is hanging there and in it's semi-final position.
I use a really THICK header gasket, so I can reuse it several times..I use no flange-sealer.
But sometimes I do not install the studs into the holes closest to the firewall cuz it may happen that it becomes impossible to be able to get the header up and over it. In this case I hang the rad end, and then install whatever bolt is easiest, enough turns to support the weight, then install the firewall end stud or bolt, then go back and remove that center one in preparation for installing the gasket.
Those four end holes go into the water jackets of factory iron heads, so whatever you put in those holes, that's where I use the sealer.

Happy HotRodding

Edit,
do not cut your gasket until the header is hanging on the EndStuds. Some gaskets come pre-cut for this.