The most difficult? Unplugging all the various sensors and wiring harnesses.

For me it has always been juggling the transmission (RWD) out and back in, clutch alignment, etc.

Like-for-like or putting a different engine in?

There’s no real answer to this question, as it varies by vehicle.

By far, my biggest pain in the ass has always been the hardware fastening the downpipe to the manifold(s). I can pull a small block Chevy in less than 3 hours without breaking a sweat unless I’m messing with rusted manifold studs and flanges. If the replacement engine is getting headers, I’ll Sawzall the pipes and toss the stubs in the bin with the manifolds. But if I need to reuse the manifolds, I might add a couple of hours, either protecting the existing studs and flanges, or drilling and retapping the threads after the rusty bastards snap off.

Damn, I sometimes hate American iron. For some reason, the exhaust manifold studs/nuts on my daughter’s '05 Camry haven’t rusted in 18 years and came apart with no drama when I replaced her entire exhaust system a month ago.

When you have someone doesn’t support you or annoyed coz of doing it.

The strain on your marriage.