Installing headers -- While engine pulled or after re-installed?

My preference is lay the headers where they belong, with the plastic bags covering everything up to the flange, as best possible (ceramic), lower the engine without Bell etc.
Sit the engine on its mounts with suitable spacer between pan/steering gear to hold it up, all bolts in but loose.
Use a front/back header bolt in loosely each side, check clearances.
Install bell/trans, leave trans mount bolts loose.
Install the "tight" header bolts first, ie 2nd one back, 2nd from back.
The hard ones first, - get the header close to the head but not tight.
If you need clearance somewhere, shift the motor with a bar, jack, 2x4 as best you can, THEN tighten up all the motor mounts, the bolts thru the motor mount brackets/block, trans mounts, always looking to adjust when nec.
Sometimes shims, or a thicker biscuit from same year van/p-up is nec to raise engine.
I never hesitate to use a ratchet strap around the collector/trans or T-bar, to encourage a little clearance, and I've been known to use a fence post up the collector, with a ratchet strap around it, and the rear spring .
It can be very challenging, but I NEVER let a set of headers leave with a leak or a rattle, my mentor/customers/me don't permit that.
Leave the bags on as long as poss, and clean handprints before fire-up, or your prints are there forever, lol.
That's how I do them, big or small.
I usually work alone now, it works for me .
Here's a chart showing where the engine should be, by spec.
https://www.ttiexhaust.com/Footnotes/FN_39.htmGood luck .
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