I think your friend has a pair of 40mm dellorto's with 36mm chokes fitted, he might have confused the choke size for the bore size. 36mm are quite big although they would suit a moderately worked A15.
If they are indeed 36mm Dellorto's then it sounds like they are the DHLB model which have been bored out. In that case they have no off-the-shelf manifold to suit oval port cylinder heads (which are the cylinder heads commonly fitted to the A15/some of the A14s).
You will need to establish whether they are the typical 40mm DHLA format or the more uncommon 35mm DHLB (bored to 36).
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I may be showing my total lack of knowledge here but would like to know if it's a total waste of time?
If they are the DHLA format, it is not a waste of time if you are pursuing a performance upgrade. Your fuel consumption will increase.
If they are indeed DHLB then yes you would be wasting your time as you'd need to custom fabricate a manifold to suit. If you are wanting to stay with the DHLB but not spend resources fabricating a manifold, your next option would be to resort to a round port head so as you can then use an off-the-shelf manifold to accommodate the carburetors.
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Also, if it is something worthwhile what manifold/linkage kit would be best suited?
An oval port manifold would be what you need, available from several on-line shops including eBay (which tend to be quite pricey). The Bprojects ones are quite good although even they have some minor issues with their linkage gear. These manifolds only suit the Dellorto DHLA, Weber DCOE and Mikuni PHH range. The Dellorto DHLB (which could quite possibly be the carburetors you are speaking of) will not fit these manifolds.
The redline kits are overpriced for their quality, I suggest you look at some specialist linkage kits that are of better quality. Although redline will do the job, it is common for the arms to snap and it is common for them to loosen and go out of synchronisation.