well, it sounds like you've got a good foundation to work with.
personally, if i were in your position i'd keep it small block and go find a decent running 318 or 360. you could probably even find one somebody's pulled out and selling on the cheap. maybe throw some parts at it if you're feeling frisky- cam, intake, 4bbl, **** who knows maybe you find one that's already nearly there.
or, rebuild what you've got. just have the machine work done and do all of the assembly yourself.
get it back running and driving and then decide if you want to go big block or build a snotty small block on the side while you're out enjoying the car.
i missed your line about ROI before. you're already at 14K which doesn't leave a bunch of meat on the bone if you're looking to flip it. i'd say a non OG big block 67~69 nacho is solidly a mid to low 20's car. maybe you find just the right buyer and you see upper 20's
but let's do a little back of the barroom napkin math vis à vis the chonk block: the exhaust (headers) is the big one, you're looking at 1K there no matter how you wanna slice it. unless you're incredibly lucky and stumble on some used tubes you're coughing up dough. same on the transmission, because it's BB specific you've gotta find that piece. automatic is the path of least resistance, but again you'll need specific parts for that-- press the easy button and get an aftermarket shifter and check that off the list and watch $250~300 fly out the window, so there's another 1K out the door. so that's 2K easy money in addition to all the other stuff...
but you want to go manual? okay, you're gonna need a whole bunch of A-body specific parts, all of which are rare as rocking horse **** and cast out of pure unobtainium, or you pivot and run something aftermarket in the swing pedals and bellhousing department which is maybe slightly less, but requires modification and fabrication to the body or pieces themselves. then you cough up for all other stuff you need: flywheel, clutch, transmission itself and a shifter. so all of that is easily 2K at the end of day.
so, you're at 14. snap up that 4K motor and then add 1K for exhaust, 1K for trans, and let's use a nice round number of 1K for all the install stuff just to keep it simple and you're at 21K. basically what the car would likely sell for.
