When electronic Throttle Body Injection (TBI) and Multiport fuel injection (MPFI) systems were first introduced in the late 1980s and early 1990s, fumigation system were pretty much the only means of converting those engines to propane operation. Fumigation conversions worked reasonably well on TBI engines but required increasingly more complex propane system controllers as gasoline injection system because more sophisticated.
The main problem when using fumigation systems with MPFI is that these systems were often designed with long intake runners to improve engine torque at certain RPMs and were never designed to convey a fuel mixture. While they did a great job of improving torque, they also created a huge volume of fuel mixture in the intake manifold while supplied with propane from a fumigation system. These manifolds are designed to supply only air to the individual cylinders and gasoline is injected directly into the cylinder's intake port. Because fuel metering on fumigation propane systems is done at the converter, this huge volume results in a large time delay (known as fuel transport delay) between the time the converter adjusts the fuel mixture to the time the exhaust oxygen sensor can measure the O2 in the exhaust.
Besides mechanical problems in the engine, even just starting an MPFI engine with a wide open throttle can cause a lean fuel mixture backfire, which can destroy the intake manifold. Franz Hofmann extensively discussed this type of failure of GM, Ford, and Dodge engines in his old web site under backfire issues. The best solution for this problem with MPFI engines is propane port fuel injection. In this case, propane is introduced at each intake port (near the gasoline fuel injectors) and individual solenoid valves sequentially control propane injection. Propane port fuel injection also eliminates the large restriction in the induction system caused by the mixer. While not being a bolt-on conversion like fumigation, port propane injection eliminates the need for specialized adapters to fit a mixer upstream of the throttle body. Unfortunately, EPA/CARB requirements are model-specific for propane conversions so EPA-certified universal systems are not available for North American vehicles.
While it used to be possible to bolt a fumigation conversion onto an EFI engine, it is impractical today because the fumigation controllers for injected engines are obsolete. Just as well because, besides the high probability of having to replace intake manifold(s) from backfire failures, fumigation systems were less fuel efficient than injection systems. Universal propane injection conversions are readily available, even for the early TBI engines, at a cost that is similar to the old fumigation conversions.
If you consider that a universal 8-cylinder injection kit (ie, Technocarb's SVIS kit) costs $2,421.43, the tank costs upwards of $800, the fuel lines and filler valve cost around $400 (depending upon the vehicle), the hardware alone costs over $3600. There great deal of work required to figure out the wiring harness, fabricate brackets, and create the propane fuel map for a universal kit. All of this work can easily take 20 hours or more to do properly. If you assume a shop rate of $90/hr, the cost of the most economical injection conversion starts at around $5500 and goes up with more expensive systems and/or bigger tanks.
If you find a conversion shop that is charging significantly less, you need to wonder where the costs are being cut. There is not much the conversion shop can do to cut the cost of the parts. In Canada, the current propane installation code requires Annex G certified tanks which pretty much eliminates the reuse of used tanks. The only place to save is in labor hours and this is not where you want the cuts. A fast job can end up costing you much more in future routine maintenance and repairs. Ask the installer if he is putting the level of care in the conversion that was done with the SVIS Impala conversion.