The question is what you are spending. On a rebuild, you should be using new pistons and rings. Honing the cylinders. New bearings. Rebalancing the crank. New valves, valve springs, retainers, keepers, and seals. new timing chain and
oil pump.Probably a new cam, lifters and maybe pushrods. A new valve job, maybe decking the block, head and align honing the mains.
You could just rebuild the 390. The Edelbrock heads are around $1300 or so in stock form. The intake is another $300. An off-the-shelf carb could be another $300.
However, the Scat crank and rods are not expensive at $750. The BBC rods are $430, or $520 with better bolts. Boring the cylinders is around $300 I believe. So for $1500 the motor is increased in size 15%. If the block can handle the extra bore, going to a 428 would be a couple of hundred more.
A Stage III head would cost an extra $1700. A hydraulic roller over a flat tappet would add around $500.
A well done Cobra replica is more exotic and expensive than a '66 Galaxie...does it deserve a better motor?