For that size poster, all vector, I would do 1/4 size. DPI means absolutely nothing with an all vector PDF. If your printer does not understand this, find another printer. The PDF can be scaled at print time from within either Acrobat to the RIP, or from within the RIP software.

Convert all text to curves. Save this copy under another name so your original can be edited should need be.

I just produced a sample PDF of your size poster at 25% scale, pretty much everything on it has some sort of transparency. I can upload to my server if you want to examine it.

I have never worked with a printer who did not expect a scaled PDF--once PDFs became a norm for hi-res posters. Before that, I would send pure PS files at scale. Largest banners I ever made were printed 50' wide and the scaled working files were about .75 gig.

Take care, Mike