I'm not 100% sure these will work yet, but I've found what seems to be the only kit that might work, it's ARP #400-1403.

I was able to pick up the ARP #400-1403 kit this afternoon from my local speed shop. These studs have a 3/8-16 threaded side for insertion into the head and a 3/8-24 for the 12-pt nut side and a raised "stop" in the middle of the two threads. The length of the 3/8-16 portion is 0.550" and the overall length of the stud is 1.670". The depth of the threaded hole in the head is 0.830", so these studs do not bottom out in the head, but I don't think the standard header bolts do either. The standard header bolts are 1.000" UHL and when you factor in the thickness of the header flange plus the gasket, I would imagine the length of thread that is left is very close to that of the thread on the stud. I also happen to have a spare 525 (without headers) sitting around so I test fitted the studs in the head and they seem to fit fine. The key is going to be whether I can get the 7/16" socket in between the tubes and I won't know that until I try it.

For me, the two bolts between #2 and #4 are the ones that give me the most grief. They are just about impossible to get the thread started. And the end one forward of #1 because of the oil filter lines.

My current header bolts are 1.25 in with a 3/8 12 point cap and found that a long extension and a swivel socket works the best and #2 & 4 I use a 3/8 ratchet wrench

Glad you got it figured out. By the way, I never did attack those trim tab cables on my boat. I just took it somewhere to pay someone else to do it. (lazy ass evidently)

Thanks. FYI, I measured the protrusion of the standard 1" header bolt below the header flange and gasket and it's 0.575", so the header stud is pretty much the same. Can't do anything more on this until end of next month when I remove the shrink wrap. Thanks to all for input.

their proud of those things, do they have the allen head in the end ?

I just bought stainless 3/8 header bolts with a 5/16 head. Should be no trouble getting a socket on. Maybe you could use those on the inner holes?

They should be; they are definitely quality parts and they're priced accordingly. They do not have an allen head in the top, that would be ideal. Now should they be installed hand tight until they hit the stop in the center or do I have to torque them in as well? Because they don't bottom out, it would seem that if I torque them in, I run the risk of damaging the top thread by jamming the stop into the crown of the hole.