340 compression ratio

I think the 6 in the posted vin should be a G? which would make the car a 318 car. So the engine is not factory to the vehicle. On the front left under where the cyl head bolts on there should be a some letters and numbers, like XW340RXXXXXXXX what are those numbers?

The 587 head like Rick said above is a small valve 1.88 head by rule. Assuming the heads belong to the engine the 587 head it is likely a 73 340 engine. This was a low compression 340 in comparison to the earlier 340s 8.5 cr compared to 10.5. It also had a cast crankshaft so look at the balancer and see if it says for cast crank only on it's face, but generally speaking it is not flat from the ring see picture note outer offset ring with writing.

With that said if anything was changed it could be anywhere on the charts. The best thing to do is see using a bore scope and look inside a cyl and see if you can read anything off of a piston. many times the part number is stamped on the top LF2316 etc. and the bore size will be also. .030 .060 etc..

This information will likely go a long way towards getting you close to what you seek?
The only way to really tell is to pull the engine apart and mic and measure everything.

The camshaft is an Erson best I can determine and here is a link to some information on it.
http://www.forabodiesonly.com/mopar/showthread.php?t=29259