Mach 1 cams in B ports dont do anything to help. I asked that question on SVTperformance when I was learning more about 4 valves.
The only swap kinda like this that I heard works is taking some 96-98 B port Cobra intake cams and swapping them into C port Mach 1s, makes and extra 15whp with no torque loss iirc.
Given the cobras are rated 15hp more than your Mark, and only the intake cams and intake are different between the two, Id look around for an intake and the intake cams from a 96-98 cobra.
Plus B ports are already large enough, their problem is velocity. Making them bigger by porting them will only hurt. The valve bowls are what needs help, so unless he had those worked on, nothing in the deal would be worth it for you.
First: Stacie, 1996 Tbird LX, mild build NPI engine, PI cams and intake, Long tube headers, Jmod.
Second: Ellie, 1989 Tbird SC, r̶e̶s̶t̶o̶r̶i̶n̶g̶ ̶t̶o̶ ̶b̶e̶ ̶n̶e̶w̶ ̶d̶a̶i̶l̶y̶. off the deep end
Third: 1997 Dodge Ram 1500, thirsty daily.
Fourth: 1990 Tbird SC, GT40 5.0 mated to m5r2, BBK long tubes, motor out the hood, drift car.