Different ways to get longer pushrod tubes. I keep always a set in stock that is about 10mm longer. At first you can underlay the tube with another tube seal to get more distance, then, if you need pretty long tubes, take 16 and make 8 of them, by cutting and soft welding. Other possible way is to get them in aluminum as a longer version. There's a shop producing them in aluminum, never used them, but they are existing here!
A rod of 5,5 inches makes a ratio of 1,74:1 that is still pretty high! Between 1,65-1,7 is optimal. With the 5.7 inchers you'll end up with 1,81:1 and that is way too much! If you would have an 88mm crank or more, then the decision would be ok!
I'll give more informations to the heads when more info is there!