The stock compression chart list it at 10.5:1 now, but based on 70cc I don't see how. If the heads are 70 cc as advertised your compression will be around 10.0 as is, using std 400 pistons .010 in the hole and a.040 head gasket it sould be about 10.5
You can get a cometic .074 gasket and get down to around... 9.5:1 I think.
Cheaper than dished pistons but the gaskets ain't cheap.
I have some pretty good TRW std 400 pistons if you're interested, run very little. I have no use for them. Most 400 blocks need punched out.
You could do a head swap for bigger valves, and 72-75 cc chambers if you want the compression down.
But I'm sure there are people on this board that know much more than me on reworking 389's though.
Can't really help you on cam selection.
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