I agree with DJ's comments 100%.
If you've got the bits needed and the motivation then go for it, disregard those who criticise just for the sake of it.
it's not for the sake of it, it's for the big picture that is era racing and preserving the bikes, certainly from my perspective.
Okay so go ahead and use a 75 frame in pre 75. No big deal.
Okay what about 75 tm250? Move the shocks back and it will clearly fit in looks wise to the pre 75 era as well as not out class others in the class.
What about. 75 Maico then? Same same.
75 Cz? Closer again.
76 cr250? Move the shocks back and pre75 here we come.
My 75 rh is basically a 74 with forward mounted shocks. Surely if I retro fit it to 74 specs I can run it pre75?
Let one slip through then there is a base for the next rule stretch. It's not about being a rule nazi.
As far as the pre78 only thing dj, I think common sense would prevail and there would be no issue if the bike was run in pre 78.
I totally agree with your sentiment, but...
If a 75 model bike is modified to 1974 specs, then it IS a 1974 model according to the rules.
I mean, if it looks like a duck, quacks like a duck, etc...
We tie ourselves in knots over the rules because we talk too much about the process, and not enough about the end result.
How can a 2012-built replica frame be OK, but a slightly modified 1975 frame not be? We need to step back and look at what is being presented, and then ask "Does this bike have an advantage over the 1974 model bikes?" and "Does it fit the era/could it have been built in the era?"
When the answer is No to both questions, then the bike should be allowed with no grizzling and not even a sideways glance.
I don't want to go down the path of Dutch-style "twin-shock" bikes, because it's a travesty to butcher old bikes to suit a historic racing category. However, while we have a class structure that makes certain bikes uncompetitive in their proper class (eg: virtually every 1975 model in pre-78), it is inevitable that people will look at whether their bike can meet the earlier era rules.