69 long bed header clearance question

[Helushar]

I recently bought a 69 long bed GMC that has been sitting for 5 years. it has a 400 small block with a t400 transmission. i need to replace the exhaust manifolds and have decided to go with headers instead.. SO my question is, since a lot of 350 parts will bold right on to a 400 will 350 long tubes bolt right up or will they have to be specific 400 headers and if they do have to be for a 400 will there be enough clearance to put on my engine.

i realize this mite be a dumb question but this is my first rebuild and regular parts guys around here cant answer this for me

ty