With the TPI manifold I believe the 84-85 manifold bolts up to early SBC. On the later manifolds the 4 center bolts are at a more vertical angle going into the head but the holes and flange faces where the bolts seat can be modified to fit. This shouldn't be too hard. The VORTEC engine has the ports moved up higher and slightly different dimensions.
While you are at it you can try porting or siamesing the runners to improve flow for the 400SBC.
If you are going to put injectors into a non-EFi manifold a single plane manifold is easier because the runners are at the same height going into the head. They produce good top-end but the bottom end torque and idle might not be as good.
I have purchased a Holley/Weiand "Stealth Ram" for my 383 but am yet to fit it. It does require cutting a holley in the bonnet(or hood for US citizens).
BBK have a nice TPI manifold that should produce good torque, with its long runners, without restricting the top-end.