I had an RA28 a few years ago that I put in an 18RG. It was heaps of fun and sounded great with the twin carbies. This is a cheap option as you don't need to worry about electrics, fuel or fabricating parts, just chuck her in and sort out throttle linkages and exhaust. All up cost for me was about $1500. I didn't get an engineers certificate for it, thinking that it was the same engine just a head change. Ignorance is bliss.
I now have a RA28 with a 1G-GTE. I did not do the conversion, but the receipts certainly add up. But on a brighter note, you can pick up another engine for about $550. Almost half of what I paid for the 18RG.
I have one pearl of wisdom to pass on though. Figure out what you think it will cost you and budget on spending double that. It is always the little things that send you broke.