i have a 1980 monti with junk open gears. is there an easy way to to turn this 7.5 in rear to a posy  

NO,and I dont use the *** things cause they are too break prone and wont hold under any heavy load pressure.There are rare cases like the 78'or79' chev mal. that made its picture in "ALL TWISTED UP" or whatever they called it in CC back in 95'or so,but 7.5 rears where never built for that,and posi-trac units that small cant take the extended abuse on a normal.I had 3 in mine and they just wont take long term abuse.Dont get me wrong if you have a stock sbc and ur lookin to get around in the winter there fine,but there not for doin 1/4 mile burnouts or the likeness.If your determinded to keep the stock rear axle,Id use air bags to hook it up.Most guys wont go that way,but the results are fukin awesome once you get it set up,and it rides better.  

if there is no way to make the 7.5 rear work is there an easy change over,or do i have to get an older 12 bolt and have it welded to mount to my coil springs  

The 12 bolt or a custom 9"ford are the best options.