V6, 3.3 L, 200 CID Engine
 

can i use a V6 3.8L 229/231 CID Engine for my 78 GP ?!

if yes : how can i find one of them and price?...
if no : which one of GM engines what are under 4L is best item to use on a 78 GP ?

1`st off, the 3.8L 229 is a sawed off 305, so it`s chevy based and has a chevy trans bolt pattern and the starter on the pass side. The 3.8L 231 is a buick based v-6 and would prolly be a better choice for a pontiac car. The 3.3L is also a buick v-6 but are hard to find. I think the 3.3 is front drive only.
I think all 229`s are carb versions and rear wheel drive, while the 231`s started as carb but went on for a long time and come in lots of versions so, you`ll have to be carefull which one you use. Some are made for front wheel drive only.

Over 200,000 Grand Prix were built for the 1978 model year.
Many of them had the 231 Buick V-6 from the factory.

If your GP already has a 301 V-8 (fifth digit of VIN = Y or W), a 231 cubic inch V-6 will be easy to bolt in. Only the engine mounts may be slightly different, but easy to change.

231 V-6 Buick, VIN code A are plentiful.
In America, you can get a running one for $100. A good one with known low miles, you could get for $200, if you bargain well.

Some 1978 Grand Prix had a 305 V-8 Chevy engine from the factory. Fifth digit of VIN is U for that engine. If it has that engine, I would try to find a 229 V-6 Chevy to install.
Chevy did make a 200 cubic inch V-6 in 78-79. That is very uncommon to find. It was installed in Monza, and in very few Malibu and El Caminos. VIN code M = 200 cubic inch V-6