The short answer?
Don't do it.
Assuming a stocker 307, the HP rating (I can look this up) was SAE gross in 1971 - to get to SAE net, multiply by 0.73. This gets you close to SAE gross - about 180 HP. To get this to dyno flywheel HP, multiply by 0.80 - this gets you down to 146 HP, which is pretty close to reality for a small-valve, non blueprinted 307.
To get this big hunk o'meat in the Opel, you're gonna have to cut A LOT of metal away - metal in areas where your feet go. You'll also have to:
a) Reinforce the crap out of the substructure of the car
b) re-design the cooling system to reject the V8's heat load
c) have a custom rearend made to accomodate the additional torque.
d) skip headers - there's no room
e) upgrade the braking system to handle the extra heft
f) ditto for suspension
Where you're done, you'll have 500+hours in the swap, a good $4000, and a design compromise. Meanwhile, a guy following Rally Bob's advice will take the same $4000 to build a stock-block Opel in less than half the time that handles better, and;
Is faster than your V8 opel.
Many moons ago, I used to street race here in Detroit (won't say where) and this discussion helped finance my way thru college. It's all about power-to-weight; when you make the move to the heavy V8, you start the dread weight spiral. In the case of the V8 Opel, you have to make another 30HP to be at par with the 145 HP 1.9 liter...
I took this discussion to the street with a 2.3 liter Pinto. Eventually got tired of the argument (Yes, it IS a four cylinder) and raced folks with the hood off and a velocity stack on so they could easily see it's not a V8. They still raced - and lost. This includes the guys who stuffed 302's into Pintos.
Read Rally Bob's stuff, and spend the time/money there. You'll be faster, handle better, and have a car that retains its value better over time.