Hi everyone from England.

I have this '69 barracuda which has a sweet-running 1978 360 in it: http://www.freewebs.com/1969plymouthbarracuda/

I want to change the oil but don't know what the correct spec and viscosity for the engine is.

Can anyone help?

Cheers,
Barry  

Most engine builders will tell you to run the thinist viscosity oil possible that still gives you adequate oil pressure for the type driving you do. Oil selection is also somewhat dependent on climate, and you don't mention what part of England you live in.

Generally, a 10W30 or 10W40 should work well for cool weather, but for high heat and aggressive driving you might consider a 20W50.  

Schaeffers supreme 7000 20-50.
It also says right on the bottle -For use with flat tappet cams.  

Valvoline 20w50 racing oil is a great product, it has a SL rating which is good for flat tappet cams as well. Also Mobil 1 has a 15w50 which also has an sl rating and can be used with any cam.
Personally I like to mix these two products 50/50, with the synthetic you get instant oil pressure, and the valvoline racing oil gives you added protection for when you like to lean on it.