In order to provide the needed strength and stability for the motor there needs to be a tube from top tube to bottom tube. The tube could have angle brackets welded to each end and held to the top and bottom tubes with U bolts.
A lot of people are using this type of mount on OCC's and bikes like that. I haven't heard anything bad about them yet, and they've been out for a while.
As far as a jackshaft goes, that's gonna be a personal choice. Someone on the forum might know a way around your jackshaft dilemma, I got nothin.
fatdaddy.
That was my social nitch in HS. What did it get me, two year books. Never took shop, now I read and watch welding how to on youtube - thankyou what-its who created the youtube!