merging two files

Shell Programming and Scripting
1

file1.txt

1 2
10 11
56 57
7 8
43 44

and let's suppose that there is a file called file2.txt with 100 columns
I want to produce a file3.txt with columns specified in file1.txt in that order (1,2,10,11,56,57,7,8,43,44)

Thanks!

2

Post sample of input and output...

3

file1.txt

1 2
10 11
56 57
7 8
43 44

______________________

file2.txt

1 2 3 4 5 ....................98 99 100
1 1 1 1 1 ..................... 1 1 1 
2 2 2 2 2 ...................... 2 2 2

________________________________

the desired output will be

1 2 10 11 56 57 7 8 43 44
1 1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2 2

i just put 1s and 2s for simplication.