Neither of those ideas work very well but some people have done motors that way. The low compression 400 pistons in a 451 is an old trick. People used to do that 20 years ago when there weren't 451 pistons on the shelf. It makes a bit of a mess since the 400 pistons are cast and are really heavy.

The Ford piston doesn't really work unless you run a super tiny camshaft since the valve pockets are all wrong for a Mopar.

My advice is that if you don't have any money then just put the project on hold until you have some cash to spare. Watch the for sale ads and maybe you'll get lucky with someone selling some used pistons or an old shortblock.