Try Alex Exhaust -I got mine there for $90 installed. They just took a W-body mandrel bent DP they had on hand and bent the top bend a little more to fit the Bonneville. All Series II Bonneville'* have the same DP, at least 98+, and possibly 95+ depending on the bolt-up setup to the rear mani.
Also, there is no u-bend to deal with on the Bonneville so no worries there.
But if you want to monkey with the exhaust order a HF cat convertor on ebay and sell the original to dang near cover the cost of the HF cat and half of the Alex Exhaust DP. Between the DP and HF cat you should see around 10-15 HP, given the car is in good running shape otherwise. Should definitely support the 3.5" pulley.