rear gear change advice

[JOZW30]

OK, so I had my rear differential rebuilt a couple of years ago and I put 3.73 gears in it (it's what the car was born with and I wanted to go back to original), and I really have regretted it as the car now runs hot (doesn't boil over, but consistently runs about 200 degrees). It never ran hot like this before the gear change.

I've played with the timing (even had Eric redo my distributor and carb since the rear change), new radiator, water pump, thermostat, fan clutch is working properly, etc..

I really can't take the car on the freeway any more. Just basically a bad decision I guess.

My question is, I think I've got above average mechanical skills, if I read up, get the right tools, and follow the manual, can I change the gears out myself? everything else should be new. Hate to yank the whole rear and take it to a shop.

Thinking about 3.23's or 3.42's.

Used to be able to hop in the car and just enjoy it, now I look at the temp gauge every two seconds..........

Thanks!

Joe