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I did convert a '74 Manta from automatic to manual with decent results, but there are some issues:

-The automatic needs a larger transmission tunnel on everything except the GT- this means more room for the larger 5-speed or easier axcess to the linkage for the 4-speed.
-The 4 speed shifter can be made to come out the same hole as the automatic shifter, and a generic square boot will fill the hole in the console nicely. (I may have had to use GT linkage, I can't remember for sure.)
-The automatic cross member can be used with the four speed if a small (@6"x6") adapter plate is used to set the automatic rubber mount @5" Back from the holes on the manual transmission.
-Plenty of room for even the S-10 5 speed transmission with almost no clearancing and a new cross member.
-The pedal will hang right in, just be sure to get both the clutch and the brake, as well as the shaft they mount to, from the doner car.

-The transition piece used to attach the cable housing to the firewall intersects the firewall at a bend in the firewall and is welded on. Removal from the doner car is only simple due to the high levels of rust common on some of the cars. Reattaching it to your firewall is also difficultboth in lining it up and in the actual method of attachment. Attaching this piece securely enough to work safely and dependibly wil make this mod almost impossible to reverse in the future if you change your mind. I ended up getting a 4" piece of 4"x3"x1/4" angle iron and welding on to it the appropriate sized tubing to get a stiff enough joint.
-Mantas have the cool firewall insulation glued onto the firewall. There is no good way to replace it an have the job look anything but homemade. Maybe this isn't an issue, but for some people it really is.

The automatic really isn't too difficult to repair. I was once even able to find a "shift kit" from a place in Italy. Remember you are looking for a Fiat Trimatic on most of the Opels and some of the last few had a TH2xx designator that was the same as the Vega.

I really wish I had the $ to try to bolt up the 4-speed automatic with overdrive from a Catera. Mmmm, overdrive!
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