I am swapping my 229 v6 for a 350 in an 81.
There is 5" difference in fly to motor mount measurement.
I have been told that I can swap sides on the frame mounts?
There are SEVERAL bolt holes in the frame, but when I switch sides I
can't get all the holes to line up.
the closest I have got is 2 bolts in each frame mount.
Can I set the motor in, line it up and tack weld the frame mounts, lift the motor back up and weld them in place?

 

My understanding is you don't swap sides, you move the mounts to the forward holes, keeping them on the same side.
You should be able to weld them in also, but they will bolt in. Put them together on your engine and set the negine in bolted to the bellhousing, then you can see where they line up.

 

I will try that, thanks.