Hey AM Humping a sandcast motor with ya mate! Hmmmm That's a first, and I thought the kiwis were sick little puppies. Yuz musta bin drinkin tooooo much fosters. I told ya about drinkin that fosters swill. Affects the mind
The correct way to put a motor back into a frame is to chop the top side bars off. Don't worry about scratchin the paint cos ya paint the frame after the motor is in. A bit of overspray I agree but that's nuthin to the trouble you have welding the tubes back in.
The thing you really have to watch is the oil filter. I broke 3 early round style filter housings trying to get the motor back into my frame. No worries though, I worked it out in the end. I cut the offending frame downtube and got the motor home eventually. Only thing I got to work out now is where I put that piece of downtube.