The motor mounts and all will fit. The 91 motor is a one-piece rear main seal motor with center bolt valve covers. They're pretty close to being the same other than those differences (rear main seal and valve covers) and I think you can make an older manifold fit, but I've never tried it.
If it was me, I'd get a new manifold for the newer style heads.
The other option would be to keep the tbi...as long as you have the wiring harness and ECM. If not...well..then a new manifold isn't all that expensive.
