Unfortunately...There is no "plug & play" LongTube Headers for a 302 powered MN12..
There are options though..
has a Sticky on it here: 5.0 MN12 Longtube Headers Installed (DUW)
He used Ceramic Coated Longtube Headers..Like the ones found here: Mac TFA793 Ceramic Coated Longtube Headers
If you're on a budget I would reccomend the non-ceramic coated version found here: Mac TFA9358 Longtube Headers
The M5R2 Manual Transmission is not as wide as the AOD..Which gives you slightly more room to work with..
Regardless of the Transmission you choose..You still have just 2 options installing these Headers:
Raise the Engine slightly with 3/8" shims on the Motor Mounts..So the Headers clear the K-Member..
Which includes beating your Floor Pan with TLC..
Keep the Engine at the stock height..Which includes cutting and beating your K-Member..
If you're running the stock 5.0 HO MN12 Intake..The 3/8" shims shouldn't effect your stock hood clearance..
If you're running an Aftermarket Intake it's likely you're going to need an Aftermarket Hood..
You don't need the "Steering Shaft Mod" if you stick with these Headers and a 302..
Going with the 351W..You surely will have to do the Steering Shaft Mod..Here's a link: Steering Shaft Mod
The "Steering Shaft Mod" opens the door to many Aftermarket Headers ..
The 351W will almost certainly require an Aftermarket Hood if you stay EFI..