There is a product called "HYLOMAR" it was invented by Rolls Royce for sealing their heads, never hardens. in the turbine industry it was the only sealer they would use on high speed gear boxes. I have been using it since I discovered it about 15 years ago. You could probably assembe heads without a gasket and they would never leak although I have never tried it but someday may.
It is very labor intensive to spread though. Marston has a brand of it and I found two German cans of it on a job although I cannot remember the original manufacturer from the firt time I discovered it but I have removed heads that I have used it on and you can scrape the stuff off and roll it in a ball like bubble gum and then nearly re-use the stuff.
It has got to be the best product discovered by me in my 35 years of working on machinery.

55 Vicky & customline
58 Rack Dump, 55 F350 yard truck, 57 F100
59 & 61 P 400's, 58 F100 custom cab, 69 F100, 79 F150, 82 F600 ramp truck, 90 mustang conv 7 up, 94 Mustang, Should I continue?