It's doable, but going to be very pricey. I would just have a custom Y pipe made. We did that on my brother-in-laws 355 SBC '82 Camaro Z28. Then ran a single 3" pipe back to a Flowmaster muffler, dual 2.5" out. Sounds good and runs awesome.
383 Speed-O-Motive stroker kit with Holley 750DP, RPM intake,Proform HEI dist.MSD6a box,World Products Sportsman II heads, ported and polished with 2.02/1.60 valves,64cc,TRW forged pistons with 10.7:1 compression, Comp Xtreme 284 cam,Crane pushrods, Comp Cam 1.6 full roller rockers,ATI damper, 40 series Flowmasters, 4.11 gears with TH350 and Hughes 3000 stall.Powertrax NoSlip Locker