What about isky 8005a ?

my vote

I just found the Isky recommendation in some older threads... I'll take a look. Thanks guys. I feel way better about a recommendation here than the tech line at Jegs, though I think they tried their best.

Can the Isky springs utilize the stock retainers? If not, I already have the Comp 748 retainers and Comp 612 locks on order.

Bill, I didn't look all that closely, but I thought the stock springs appeared to be a single with a damper, which is what the 911s are as well. They look like a double but the inner spring is a wimpy flat piece. I didn't look all that closely, to be honest.

I just ordered 2 sets of the Isky 8005a from Summit.

Those Comp 748 retainers are for a single spring. I am running the 911s on a 230/238 .530 116+4. Shimmed to .060" b4 coilbind. 150/350 spring psi.
They will suit your needs fine. However, I would use a dual spring on a marine setup. Added protection incase of spring breakage.

Measure the OD of your valve stem seals compared to ID of the inner valvespring in a dual setup. Make sure you have enough clearance. If not your guides will need milled for smaller seals.

I sure wouldn't pay extra for the Isky springs when retaining the stock cams. Whatever the specs and quality are on the stock springs, they have to be about perfect for that cam and valvetrain. It's rarer than hen's teeth to hear of a spring or roller lifter related issue on the stock black motor BBC - even with 1000+ hours. New springs? Sure, but I wouldn't mess with success, especially with those stock springs most likely being plentiful and cheap.

I broke a valvespring with about 150-200hrs on black 502mpi flat tappet.

I tend to agree -- BUT - the stock springs are no longer sold as a set by GM. Individual only. $15 each, X 32 of them, = 500 bucks to run stock springs.

Heads are back from the machine shop, Isky 8005a springs, Comp 741 retainers, 612 locks. 1.875 installed height and 135 lb seat pressure. If weather cooperates this weekend I'll start to reassemble Stbd motor.

As a side note, the stock springs (or whatever it was on the heads at disassembly) were in fact double springs with a damper.