"new" 1969 Barracuda

Just keep the stock parts if you are worried about value.

The weight difference between a 318 and a 340 is insignificant.

For suspension a standard rebuild with quality stock parts, coupled with sway bars, 1" torsion bars, subframe connectors and modern gas shocks will transform the car (the addition of a performance radial in a + size….. 15", 16", 17"……. Will take this even further)

You will a need a factory KH set up (or aftermarket kit) for front discs is you want to stay with small bolt pattern.

Less expensive would be a Large bolt pattern swap off a later a-body for front discs but with this you will also need LBP mods for the rear (unless you want 2 bolt patterns on your car).