would that be the easiest way to put it in? Might as well start research now, you know? so you have to disassemble the whole top just to put it in the car?
Just research "1967 Mustang FE engine install" or "swap"(the 390 is part of the "FE" engine family of big blocks).
It's a really tight fit, even with stock exhaust manifolds. Changing spark plugs is a pain-in-the-ash. Header fit is challenging.
If you want an easy swap, 5.0L/302" small block, or a 331or 347" stroker using the 302 block is an easy, buy the parts and drop it in deal.
A 351Windsor makes a nice swap candidate, or a stroker built on a 351W block, header fit is a little tighter.
Then there is the 351C, or a 351W with Cleveland type heads(called a "Cleavor"), its do-able, most all the parts are available but header fit gets tighter yet.
Then it's a pretty big jump in physical size to the 390FE....and weight. Fe is among the heaviest of big blocks. Big ole hunk of 1950's tech and design.
Was your car originally a inline6 cylinder, or a V8 small block 289?