She’s looking beautiful. I noticed too that you also did the ‘black-out’ treatment on the dash bezels.
I did I’m hoping to find a good way to rechrome these things.
I also have an issue with the 390 it’s quiet when cold however when it warms up the lifters make noise that I can hear through the firewall. Maybe someone here has some ideas. I’ve tried a new fuel pump, oil pressure is good no valve lash when warm or cold. Went over exhaust gaskets and such . Kinda frustrating
A change of brands of oil has helped me in the past with a ticking lifter different oils come with different additive packages and they work in different ways.
The rocker shaft / rocker arms often get overlooked and installed improperly. That can cause the noise that you describe. Often the shafts are full of sludge that can’t be seen which can cause oiling problems to the rocker arms, leading to rapid wear and noise.
The other common problem is mechanics don’t know how to install them correctly. The oil holes need to be oriented properly or the rockers wear out and the shafts get galled.
Final common problem is that machinists don’t always do their job properly. In an FE with non - adjustable rocker arms the valves all need to be the same length and seated properly in order that the hydraulic lifter can operate within its proper range of travel.
There are many obscure problems that can also cause lifter noises but these are the most common in a 390. I would start by using a piece of wood dowel to determine which cylinder the noise is coming from. Then remove the valve cover and the rocker assembly for a careful inspection.
Is it all the lifters are just one? If all, I would lean toward the rod or main bearings are wrong size for the crank journals. You always have to check them when they are either polished or ground during rebuilds. I’ve had them say one size and found out to be another. When cold the oil is thick enough to take up the clearance. When warm, the thinner oil won’t do it any more and the lifter rattle. Had it happen on a 340 Challenger I owned. Replaced the rod/main bearings (down to the copper) and problem solved.
Awesome job on the restoration of that special GT!