rstang91GT

Ok, who's the A/C expert? I have a 1991 GT convertible with a leaking evaporator. When it was easy to get R12 and I could recharge it, it would dump the freon in about a week. I read the thread on heater core replacement so I guess I'm in for a real treat since they are both in roughly the same area under the dash. The A/C not working is one of only a few flaws on a real beauty that has served me well for almost 100K miles. It hasn't worked now for about 3 years and I'm thinking I should just replace all componants and convert to R134. I'm a pretty good mechanic and very handy with tools but have never worked on A/C before. Any tips or am I crazy for even thinking about this?