Engine and gearbox on chassis first for me. Apart from having to lift body a bit higher to clear engine when placing it on chassis it’s so much easier to manoeuvre without new paint damage.
Very tight fitting engine and g/box as one lump down at an angle into body if you fit body to chassis first. Easy to scrape new paint, particularly on bulkhead above transmission tunnel.
If you do decide to fit body first, then engine and box leave the g/box lid off to give a bit more wiggle room.
Dave McD