The repros are rubbish, the bypass pipe isn't even the correct angle to meet up with water pump. You probably found that too.
The standard Olds neck used on the '79 Olds models is luckily a common part used so look for good spares.
The 45 degree version for most Olds and 79 F models was #409037 , even though the casting reads #409038 and more likely what a used one is listed as. It uses a different radiator hose to the early models disguised as a 400.
The biggest problems are corrosion of flat base at thermostat and corrosion of the bypass pipe which is cast into it. It sounds like yours isn't sealing for any number of reasons, did you check surface corrosion around where hose seals, pinholes? Also, where the pipe bends over it is ovalised so you need the clamp at the base where it also has a sealing lip. Try the widest hose clamps in that diameter, and install directly above the raised lip. I have the original tower clamps on mine, known for not sealing well, and have no dramas. Worm drive are better for problem areas or minimal maintenance.
This is my NOS 77-78 version, but can see the shape of bypass pipe and smooth surface for sealing.
Olds water neck 77-79 GM NOS 547083 (4) by
Aus78Formula, on Flickr
This is my used water neck, you can see some corrosion and pitting to the sealing area, and yet it's far better condition than others I collected. One had half of the pipe missing at the base. I cut it off, reflared the outlet and sold to someone who knew I'd repaired it.
547083 Olds water neck 403 Firebird by
Aus78Formula, on Flickr
