Opinions on low end 360?

Here is just a suggestion for thought: You might not want to mill the J heads at this point if you think there is any possibility to re-use the J heads later. If are going to put in new pistons later on, you may end up with too high of an SCR unless you already have planned what the later pistons will be and what head CC's you want to end up with later. If the new piston tops are higher up in the bore later, you can only put in so much more head gasket thickness to lower the SCR once you have milled the heads down.

Of course, if you are definitely going with the AL heads later, then there is no problem with milling the J-heads now.