The lower timing covers (inner and outer) are exactly the same between the 5 and 6M. From what I remember I used both the orginal 5M cover on my 6M when I rebuilt it. From what I recall the 7M rear lower cover is a little different than the 5/6M one around the water pump. You do need to remember to use a 7M front main seal with the 6/7M timing cog (the ID is bigger). The 8.8:1 pistons will give you 8.8:1 compression with the 5M head one. Piston pin location is the same. If it isn't you are screwed the pistons come flush with the block deck at TDC and the clearance is the HG thickenss (maybe .030" compressed or so?), so they can't go up much higher.