Looking to replace my 55 amp 15 year old alt with a higher output, I run a single 540 bbc , my 2 batterys seem to need toping off with A batery charger sometimes. Would i be safe with a aftermarket 105 amp?

Not sure what you mean by safe. I have been using a 105 amp for two years without issue. I only have one battery. Perhaps you are concerned it may not be enough?

Most people worry that the higher amp alt. will over charge, charge too fast and ruin, or otherwise wear out their batteries. The voltage regulator takes care of over-charging, and I've never had an issue with a battery wearing out too fast from charging too fast, but there are a few battery experts on here that I'm sure will chime in and provide some more sound advice.

Ive Been looking at delco alternators, from 55 to 110 amps. im trying not to over load my electrical system. I have found out the more amps the more HP it takes to run it, with and internal regulator, it wont let me over charge the batterys. Is this info correct? like looking at cough syrup and not sure which one is best, thanks for your imput.

I'd believe you'll have to upgrade to a heavier wire for the higher output. 6 AWG at min. possibly even 4 AWG. The alt will only put out, up to it's potential, depending on the load draw... state of battery charge. That's the regs. job. Gotta have the right rated wire so you don't burn it up. Then chit can get interesting if no fuse inline.

I was hoping for plug and play,not FIRE , im still running the 1994 55amp. looking at aftermarket .http://www.dbelectrical.com/ I think i will order the 65amp replacement.

Doesn't matter. You have to change it out.
A 55A alt can use a 8AWG wire at min. (Not sure what Merc used, but doubt it was larger than needed for a 55A.)
A 105 will require that wire to be upgraded. It's a simple 3' replacement from Alt to starter solenoid. The battey cable will handle the starter to battery circut. You'll have to check and see if you have a fuse at the starter in that alt wire. It might need upgraded too .

I will replace that wire now , thanks for the info.

I think you need to upgrade the ground wire too, never hurts to use a bigger than needed gauge of wire. I have also been told to stay away from the 1 wire set ups and to use the traditional 3 wire set ups

3 wire is my plan.