I agree with Simon - avoid the steel seal
.
I would also add that HGF on a KV6 doesn't have to be such a major job if you can find the right mechanic. When my engine noise was finally diagnosed as piston slap, I decided I wanted it sorted, so agreed to have the engine removed and stripped down. As the heads would be coming off anyway, the mechanic decided he'd take them off with the engine in situ to make the engine smaller & lighter for removal. They were off with no issue in less that 2 hours of his arrival. The replacement followed a similar pattern. He replaced the clutch and engine block and then had the heads on in no time at all. I had offered to give him a hand with the rear head as it looked like it might be a little awkward, but it proved unnecessary and he popped it on single handed much as he had done with the front head.
All in all, replacing the heads were probably the most uneventful part of the rebuild.
I should perhaps add though, I'm no mechanic, so wouldn't dream of doing it myself
.
Cliff