I'm in the same boat, I have the stock seats which I like because they are period correct. There is no ass cushion left which actually is good for me since I'm 6'2 and its enabled me to fit. The downside is that after an hour of driving its uncomfortable. I need something that sits very low and also tucks right up against the firewall, so my butt can slide forward and knees are just bent more.
The C4 seats are really nice, I believe I sat in Gary Herrig's car before he sold it, but I was pretty squashed as there was a lot more cushion.
I'm debating some Kirkey Classic Buckets with the pseudo leather button on upholstery at this point. That way I can add cushioning as I see fit and alter the layback degree.
I can't tell, but this looks like a stock seat re-uphostered, but the headrest removed looks like more clearance to move the seat back ultimately.