I am rebuilding a 2.6 litre astron motor out of my Tp and found cracks between the valves in cylinder 1, 2 and 3. The head has M3 on the exaust side and the head I looked at to replace it has M1 They are both on fuel injected heade are they different or just different time of molding? any help greatly appreciated. thanks Andy.

The "M" number is just the casting batch. If the inlet manifold stud pattern is the same they will interchange, I think maybe carby is different.
I have heard M1 and M2 were softer and more prone to cracking.
Interesting to note that both M1 and M3 are EFI heads,
If you want something different/better fit a head off a TR or TS complete with cam and roller rockers. M5-M9

tr or ts head sounds like good idea they say they have roller rockers so they may give a little extra kw.