motor stumbles after it warms up

[fixkick]

i dont think you can mix intakes.
The engine clearly has G16 casting marks in the casting

here is the 91, block, TBI G16a
see the top left there.
11am here.
see that.


no mixing up G16 with G13
the serial number is ( see that bell boss at 9am the top of that is stamped, the engine serial.
G13yxxxxxx
G16yxxxxxx
y = year, in usa.
xxxxxx is serial, its clear as day what engine block is there, in TWO places.
head codes are vastly more complex.