I had a 1977 390 rebuilt at napa in 2007.
It isn't assembled just machine work and parts.
I don't remember the exact breakdown but what I could remember is:
total bill $1800
magnaflux and resize the rods after installing arp bolts
new pistons
Moly rings
bore .029 over and line hone with torque plate to fit pistons
check block deck and crank bore for straight
boil out 2 engines from which I got the parts to rebuild one
mag crank turn .010 under and polish
new valve guides
regrind valves
check heads and resurface
new valve springs
new cam
cam bearings
bill includes the complete rebuild kit. all gaskets oil pump and the timing chain.
the rebuild kit wasn't much more than a kit for a small block chevy.
My 1.5 cent opinion:
I'd guess still around 2k at least for a good quality rebuild at a machine shop with a good dishwasher, magnaflux capability, line bore/hone mill, crank machine, and deck resurfacer.
I've seen cheaper but those guys are almost rebuilding engines on the floor
with hand tools and inaccurate block mounted bore machines from Fred Flintone tools.
I don't think theirs anything wrong or iffy about a clean careful hand rebuild and light hone if the engine is pretty close to right on.. but anything requiring actual machine work....get a shop using the right machines.
If your requiring assembly the machine shop can do it or there are guys with good skills to spec out the machine work for you, check everything, and assemble it.
don't let the lack of equipment scare you...
I don't want to discredit anyone putting engines together on the side or as a small business who get the machine work done somewhere else but done right.
there are some great guys who farm out the machine work but do all the design and assembly.
I live here in Omaha.
Charley's speed and machine is crazy busy
Mike Daley's is a great shop with all the right stuff, Mike built my 460 about 9 years ago. It cost $2500 but I had more work done than on the 390.
NaPA here in Omaha closed their shop and sold everything. My 390 is one of the last engines they did
I think... If your not worried about a warrantee... Your best money spent is with a local quality shop and built your way.
Getting my engines built my way cost me less than a paycheck over the cost of an over the counter rebuild like a 4star or a jasper.
Most thrown rods are mostly from weak rod bolts...not the rod failing itself from going up and down.
weak rod bolts stretch at tdc, one bearing shell spins around and overlaps the other and when the bolts pull the caps back tight near bdc... the rod locks tight to the crank and snap-bang there she goes
arp rod bolts are the cheapest insurance your going to buy if your going to be beating on that engine.
water in the oil from an improperly seated head gasket will throw a rod in a heartbeat...water mixes with oil and the friction sucks the bearing to the crank...bearing spins, rod snaps...
so will losing oil pressure.
dropped valves happen mostly from valve retainers or the grooves they sit in being worn... a good head man will check them and replace if needed.
Valve springs with insufficient pressure to keep the valve train parts from floating (bouncing apart) will lead to valvetrain parts failure from the added stresses.
springs to stiff wear out the cam and lifters faster.
stock springs can be used on stock cams and might need to be shimmed to bring up their pressure but performance cams should always be installed with new performance valve springs matched to the cam for the rpm range your going to run in.
My essentials for a cheap engine( if it blows up on me and I have to get another one...not cheap)
*arp rod bolts.
*new oil pump
*absolutely straight, clean and if need be... resurfaced heads and block deck.
properly checked and machined machine surfaces.
*good head rebuild
*There are some type of special alloy iron valve guides longer lasting then stock cast iron or even bronze and cheaper than bronze. I use those.
*There are umbrella valve seals made from a special material that doesn't get hard or crack..I use those because expensive positive seal valve seals seal really good...they don't let as much oil get to the valve stem
*sealing- I use Permatex aviation type (Indian Head -coal tar gasket sealant) on both machined surfaces and both sides of the gasket and allow it to tack up before I assemble. they never leak.
leaky engines sneak up on you eventually and blow up when you forget to check the levels
I use silicone where I will be removing parts for maintenance like on one side of the valve cover gaskets.
thats how mine roll