I had a 78 SWM 250 (which as pointed out is eligible cos its same as 77) but it wouldn't be my choice. Suspension, handling & brakes were great, but power is all wrong for MX. King hit at about 7500rpm means you never got outdragged in a straight line, but give me a fat midrange for MX any day.
I also had (& still have) a MK9 Pursang that I raced back in the day. Much as I love Buls (& the Mk8 was at the pointy end in its day) the Mk9 is a bit outclassed in pre78
My choices would be:
125 - RM-B, by a long way
250 - probably VB Monty, or perhaps a Beta or Villa if I could find one
500 - not really interested, but probably AW400 Maico