Yes, twin 49's= 69mm single. Twin 61's= 86.2mm single. . I just measured the factory TB- it's smaller than I was visualizing in my head. 39.5mm X 2. Measured a YF carb@ 42.6mm. So my 1990 factory TB=a 55.8mm single bore. Roughing the figures to get a ballpark, I assumed the YF flows 200 CFM. (Someone correct me here if I'm way off.) That calculates to my twin 39mm TB flowing only 344 CFM. It's somewhat more, without venturi restrictions. If 344 CFM is close, that will max out a 305cu in engine @ 3900 RPM. I may need to go up to the twin 49's you referenced. Thanks for the input.I know the stock 302/351 TB is twin 49mm...with that you can do the numbers for potential airflow. I think I came up with around a single 65-70mm area once, long since forgotten. Only reason I know is my '88 5.0 truck now uses a BBK twin 61mm which flows around a single 90mm. Yes, I picked up the unit as a blem new in box unit to go for my Edelbrock intake for the Bent-8.
Also, I measured my stock 4.9 unit, but have forgotten. I think it was 41-43mm area, not much but the stock 4.9 does not make massive power at high RPM. Talking back in early April and that has been a while.
Recently I read the earlier EFI trucks had a larger plenum volume and runner length. So you should mind that.