I only have a 2008 MOMs book
where it says only upto L models ie 74 models .
But i think they were revised recently ( not 100% sure ) to include 76 TS A models in pre 75
Either way run your 74 L model frame as in your photo and motor , but run the A model wheels , footpegs , Engine outer cases , and TM style Rack and pinion clutch actuator ( fitted to A models ) .
The 250 A model ran a different cylinder (electrofusion bore and different porting ) so probably not elidgible for pre 75 .
It sounds confusing but you have a basis of both a good Vinduro mount and a pre 75 250 racer