I realize that is a question that requires a bit more information so here goes.

I am a 36 year old guy that weighs about 230#. My commute is 25 miles one way. About 20 miles of this is on a flat MUT. I live in MI, and am not able to commute through the winter as the MUT becomes a snowmobile trail. I live in the country, but commute into Grand Rapids, which is not New York, but still has lots of stoplights and other city cycling problems. I currently make this commute on my Gary Fisher Marin (MTB), which I have commuterized with lights, fenders, road slicks, and rack. Currently, I am a nursing student, and can afford the time to make this commute (90 to 120 minutes each way). I have done a little reading here, and have not seen anyone who is commuting that distance on a FG (I am sure somebody is, but I haven't run across the reference yet.)
I have my first "real" bike, a 1989 Raleigh Technium 450, that I think would be a good candidate for a conversion. There is some sentimental attachment to this bike, but it is currently not rideable, due to a wrecked rear wheel. All other components are workable, but a by today's standards, are far from great. I want to do something with the bike, and converting it seems the least expensive way to go.
So what do you guys think? Is my commute way to far to make on FG? Is my conversion idea plausible?

Go for it.

Buy a fixed/free rear hub/wheel, so you can move to SS if you aren't crazy about pedaling 20 flat miles fixed.
That can get boring....

First off, do you understand the difference between a FG (fixed gear can't coast) and a SS (single speed can coast)? To my way of thinking, you'd be better off with a SS and find it easier to do the conversion with your old road bike. I'm assuming from your description of the commute that you don't need to change gears much due to the flat terrain, but do you have to face strong headwinds? Where I live, it is often very wind to the point that riding a bike with one gear for long distances is not practical.

I'd get a nice road/hybrid/cross bike. Is wind an issue? Keep it in mind pedaling home 25 miles into the wind with one gear. It sucks. I do it sometimes but only 7 1/2 miles.

"I am a 36 year old guy that weighs about 230#"

How tall are you, OP?

Oops. I meant to add that I am 6'. I do understand the difference between SS and FG. Part of my thinking in doing the FG or SS conversion was that if I found it not to work well, I could build up a geared hub and get back to gears.

Wind is not as bad here as in a lot of places. My ride is mostly north/south, so I don't deal with the normal wind out of the west that often.

Go for it. You need a new wheel, nothing is irreversible (I hope) and the components whose tech has increased the most are going to be the ones you strip off anyway. Headsets and brakes.... still more or less the same as they were 30-40 years ago.

Whats wrong with your current commuter? I personally would not go fixed only. Wind and hills after 20 miles can be tough. It's also nice to be able to coast, and save the legs for the hills. I just got back from a 25 mile ride which I do three times a week. Lots of wind and long not really to step of a grade in several parts of the ride, but I was only making 8 mph on the last hill. I do it for fun, and not to commute though. I don't have to work anymore. Yee Haw!

Get a $30 single speed conversion kit for your Marin, and give that a try on a Friday. You may find that one gear is not going to work, but at least you'll get that answer cheaply.

Tonight I did a short ride (ten miles or so) on my Marlin. I decided to try it in just one gear. I chose the gear I normally ride down the MUT. I may have chosen too high a gear, because it was very little fun. I also road east/west into a 20 mph head wind, in the hilliest part of my commute. Unless my road bike conversion is somehow a lot easier to pedal, I am going to have to drop back a gear or two or three. That will lengthen my commute quite a bit. I am starting to think that a gear hub might a better choice.

I love fixed, and I ride mine 25 mi in the country sometimes, but for a commute that's that long each way, I would definitely use gears. You want a touring bike, essentially, and if you're going to spend that much time on it it should be a nice one.

My friend who spent a year touring still commutes on the same bike- a nice old lugged steel italian frame with curved fork tubes, powdercoated and fitted with a triple up front and a seven speed freewheel, operated by downtube shifters. He has ergo drops, aero levers, a mirror on the left, two water bottle holders, a sprung Brooks saddle, SPD pedals, full fenders, front and rear racks, and Schwalbe Marathon tires (28c or so). You should decide on bars, saddle and pedals depending on your preference, and get the bike fitted right- but I would put together something along these lines if I were you.

Almost anything you do with the Raleigh is going to be better than riding a MTB on the roads that far, but that's a substantial ride and you should put together the most appropriate bike for the job or it'll be a PITA and you'll quit doing it. Your Raleigh frame is great; you can reuse components that you don't have a reason to replace, but set it up as a hardcore road commuter, and you'll be better served than if it were fixed.

You can lose wind-resistance by crouching forward slightly. Or you can avoid that route altogether because some routes are just too hard whichever bike you choose. ... One of the most overlooked virtues (and often avoided) of cycling is that you can get off and walk when it gets too hard or impossible.

Whatever the choice you make, you ought to visit your local cyclist shop and ask about gearing. You know like Gear-Inches, proper gearing tips and stuff. Most of em are nice, and sometimes they can give you the whole training for free. My LBS people have been great.