I have new pistons they say 9.5-10.7:1 depending on the cc of the heads. The mopar guys around here say the heads are no good and my cr will be around 8. I'm on a budget build right now cant buy new heads.
Before you worry about milling the heads you need to know where the pistons sit in relation to the block deck and what the actual cc's of the heads are. The blocks typically were machined with a deck height on the tall side of the spec which lowers the compression and the heads could be anywhere from the 68 or so to over 70 cc's. Time to start measuring your parts, then you can make a plan on what machining to do.
Also, if the heads are no good why are you spending money on them. If no good means they aren't a good design that is not true. A 75 360 head is for all practical purposes the same as the head that would have been on the 69 340 with the exception of the valve size. All the open chamber factory 340/360 heads have the same basic port size and shape and the same chamber until the 308 casting that came out in the late 80's which has an improved exhaust port shape.