Speaking as a professional, I am
NOT any fan of Selden spars for reasons too numerous to detail here. Suffice to say they will
not have the proper mast section but will sell you on what they regularly make rather than making a mast for you. They sell their own parts; they are
NOT in the refitting-older-boats-with-original-type-spars business. Most of their sections are rather oversized, clunky-looking, especially up to where they do not taper the spar till after the last hook-in shroud attachment (on your 37C that'll be about 2-3 feet above the inner forestay's attachment). They eschew shroud tangs and incline towards big slots and hook-in bolts. It's very Erector-Set-looking and very noisy. Their main business is in supplying Beneteau and Catalina with 15/16 fractional rigs using a limited number of 'stock' in-mast-furling spar sections. You get what you pay for.
I would suggest looking up actual sparmakers. My first choice would be Dwyer Aluminum Mast Co. Their biggest section may suffice. See their web site and give them a call (and tell them I sent you!
).
If DAMCo cannot do it, I can think of Charleston Spars (conventional) and Hall Spars (racing, carbon-fiber). There is a place on the West Coast whose name I forget. The Wichard group are based in or near Charleston SC and they make nice spars too. Sadly, Metalmast, Schaefer Spars, and Kenyon are all out of the business. Rig-Rite tend to be
VERY expensive, even on shipping-- I'd avoid them unless you're sure they have
an exact replacement in stock and can ship reasonably quickly and cheaply.