.039 is good quench but beware the thicknesses of head gaskets can be way off from the stated dimention (mike an old one of same type). You want the DCR reasonable enough for your octane so you can run an aggressive ign curve. On the aluminum there was a good discussion here (if I read it right) on that & that other than the weight savings off the front that there is no real magic as in more power from alum VS cast iron but you would get closed chambers and likely better ports (plus potential for more porting). Just me If the octane is set in stone (& proportioning in race gas in a DD gets old fast) I would lower the SCR/keep the quench (if you can with your piston/gasket selection) then max out the ign and ESPECIALLY get the CORRECT cam in there for your (driving) tastes