I believe offenhouser makes a dual single barrel intake for the 235 and you can get headers from Speedway motors. Tom Langdon's headers are for the 194-230-250-292. The 235 and 261 have round ports. Fenton headers are a great set. Lot of people have used them. Clifford Performance may also have an intake and headers.
Tom also makes headers for the round ported 216, 235 and 261.
http://www.langdonsstovebolt.com/store/online-catalog/#!/Williams-Cast-Exhaust-Headers-for-216-235-261/p/2486512/category=412417
Cut and paste the link into your browser.