Some good advice in here generally. Regarding flatness of the heads, Disco heads are extremely thick and it would take a crazy amount of heat to warp them. As some have indicated you can place a straightedge on the lower surface (readily available at Home Depot for less than $20 and look for gaps using either a feeler gauge and even light (if you can see any gap, no matter how small, that is not a good sign and you should have a pro measure it.

As a point of information, I have measured the crush on the LR hg recently and it comes to about .005". Gaps of less than .002" would likely have little to no affect depending on the region. LR HG's blow primarily in three regions:

1. Between the water jacket and rear cylinders (most common)
2. Between the cylinders and outside air (2nd most common)
3. Between interior cylinders (3rd most common).

I have never seen a blown HG to the lifter valley, although it probably is possible.