Here are a couple of 'recipes' for improving the performance of your engine without investing a ton of cash. I've broken it down into two categories, bolt-ons and internals.
Bolt-ons.
>38 DGAS Weber carb. It will work on a mild engine, even a bone-stock engine, but must be rejetted accordingly.
>Ported intake manifold, preferably '73-'74 smog-style. Easiest to port and get good results.
>1975 'Sprint' exhaust manifold from fuel injected Opel. You'll have to grind the triangular nub off the center pipes to clearance the intake manifold.
>Custom 2" exhaust pipe. GT's only have a 1 5/8" pipe, while Manta's come stock with a 1 7/8" pipe, yet they're rated at the same power (? I never understood this...). Even a stock engine will benefit from this mod, you will not lose torque, and it will help the entire power band. Use a perforated-core, straight-thru design muffler up front. For a GT, use resonated rear tips, and a custom 2.5" over-axle pipe splittting into two 1.75" pipes out back. Nice sound, not too loud, and looks good. A Manta can use a second straight-thru muffler (round body) and it will emulate a resonator nicely.
>Electronic ignition. At least a Pertronix unit, with an upgraded coil. Another upgrade would be a Crane XR-700, but for a few bucks more a Crane XR-3000 is a lot nicer, and you can eliminate the resistor wire in the ignition with this system. Use a Crane PS-91 coil with this, it works well. Lastly, another option for the ultimate in 'trick' bolt-on ignitions, the Compufire DIS-IX 'distributorless' system kicks butt.
>Electric fan.....much quieter, and adds a few hp in the upper rpm range.
>Tuning! Having the parts is one thing, but getting it setup right is another. Most cars are not getting nearly 100% of their part's capabilities, so setup is crucial. Consider having it dynoed on a chassis dyno. These days, $75-$100 will get you about 6 pulls on a dyno, and tell you more about your setup than a month's worth of driving on the street. If you can get 6-8 hp from tuning, then you just got more power than a 38 DGAS is worth, and for a lot less money. Not to mention, you will get better gas mileage, and save $$$ in the long run. Well worth it.
Internals.
>Bore block .020" over, install flat-top pistons. A small increase in power from the displacement, another small increase from the compression, and yet another increase from the 'newness' factor. It's no longer a 30-year old engine. Use Total Seal gapless rings on the second compression ring. Relatively modest investment but the engine holds compression nicer.
>Lighten flywheel. No power increase, but acceleration is improved. Stock flywheels weigh 22.5 to 23.25 lbs on average. You can very safely lighten to 17-18 lbs.
>Recondition the head, install hardened exhaust seats, larger 2.0 litre intake valves, and mill .050" off the head. This will increase compression a bit, to about 9.5:1 true compression. Still workable with pump gas. Don't worry about cam timing, I'll get to that in a moment. The head's ports can be cleaned up a bit. Don't enlarge the actual port area, just blend the bowl areas, and have the machinist mill about 1/4" off the intake valve guides and 1/8" off the exhaust guides where they protrude into the port. Blend the edges of the guides slightly.
There, you just improved airflow by about 10-12 cfm per intake runner, and 8-10 cfm per exhaust runner with minimal effort. If you do any more to the head, then you'd better know what you're doing, or you may do more bad than good! After the head is milled, make sure to deburr the edges of the combustion chamber to reduce the chances of hot-spots.
>Performance cam. Nothing crazy, if you are using stock valve springs (you should be), then you are limited to .425"-.430" max valve lift anyway. I usually keep it to .420"-.425" max, but that's just me. If you are not gonna rev it hard, use hydraulic cam/lifters. Less maintenance. Shoot for a custom profile, with split intake/exhaust duration. This helps the Opel's notoriously weak intake flow (note the trend? Most modifications are aimed at intake flow). For a truly mild driveable cam, I'd go with around 212-214 degrees of intake duration @ .050", and around 6-8 degrees less on the exhaust lobes. Grind at 110 degree lobe separation for a smoother idle and broad power band. Now, to correct for the milled head, have the cam guy grind the cam with 4-5 degrees of advance. This will correct the retarded cam timing, and give a touch of advance for better torque. Stock Opel cams are ground 1 degree retarded for comparison.
If you decide to put a bigger cam into the engine, expect to have to modify the distributor to get good throttle response. This mild cam I've recommended would run well with 3-4 degrees more initial ignition advance than stock, but can use a stock distributor. I'd use a later distributor too, a 72-74 model.
Anyway, just some ideas/feedback from someone who's built a LOT of Opels over the years, and has just about seen/tried it all.
Bob