red80roadrunner: I did my swap because I was told it couldn't be done and it turned out easier than I thought. My swap was done quite some time ago when the Schumacher mounts were not so readily available and I didn't want to pay big dollars for headers either.
How I got the mounts aligned was to put a straight edge across the back of the small block where the tranny mounts to the block and measured to the front side of the motor mount where it slides in to cross member. This dimension you will need on both sides.
It will be 3/4 of an inch different on the passenger side and 1" difference on driver's side from the big block measurement from the back of the block. Now if you measure the motor mount ear at the top front, you will see it is almost an inch. To mount the motor mount on the front of the mounting ears, you will have to grind some cast flash off. You should be able to mount the top bolt in this area to get your measurements. If I remember correctly, I had to grind about 1/8 th on the passenger side to move it back and put a washer in on the drivers side to get the motor mounts to sit the same distance as the small block when bolting the big block motor mounts to the front side of the motor mount ears. Use washers or metal spacers to fill in the gaps on the rest.
This will make more sense when the mounts are bolted on and you can see them in front of you. Driver side motor mount will need some grinding in the oil pump location and rear oil pump spring bolt will have to be ground down (see photos). Best to fit motor mount and pump before installing engine. You will have to cut cross member mount out side front down to clear pump (see photo). I used torque strap to hold motor down so it would not pull header into steering link. This will only be an issue with Hedman #75060 headers on an RB engine. Some people have had luck with 78030 Hedman headers but I have never tried these. I used the #75060 header because they come in 11 pieces and were very easy to fit. They did fit tight on the firewall and inner fender on passenger side of an F body though. Hope this helps, if you need more info let me know.
I would not do the swap with the Rubber crossmember bushings in, must upgrade to the cast or polyurethane bushings.