If you post a pic of the engine in your car, I can confirm it is an olds. If you are saying its a 403, then that is an Olds engine. GM put the 403 in the transams in 79 so that makes sense. The problem with the 403 being a great street engine is the relatively short 3.385" stroke, but you might consider building it since you already have it in your car.
Here's an article about hotrodding the 403, with a pic of a 403:
Notice the front oil fill tube and also the curved water tube on the front. This is a quick way to ID an olds engine.
The pontiac engine is not that much heavier than a small block chevy, depending on the source of information. I have found information here stating engine weights.
Pontiac 590 lbs or 650 lbs. (two different weights are listed? which is curious)
Small block chevy 575 lbs.
Big Block chevy 685
Olds 403 560.
Olds 455 620.
Cadillac 500 625.
The pontiac 455 is hard to find, but 400's are easy to find and you can buy aftermarket cranks for them to build up a 455.
HP is fine, but torque what you will actually feel. Torque is what does burn-outs, wheelies, and what you feel pressing you back in the seat when you accelerate. Torque a mathmatical expression/measure of work done. HP is a measure of work done over time, i.e. Torque x RPM = HP. I think "Horsepower" is a more handy marketing term, but real gearheads know that on the street, torque is king. I wouldn't worry too much about added weight as long as you are adding cubic inches too. You can stomp the **** out of most anyone on the street even with a heavy car if you have a huge cubic inch engine that makes gobs of torque at low RPM.
This is why cadillac put 500 cubes in their cars, so a 70's 6000 lb. land barge sedan deville could still be quick and fun to drive.
So build for torque, and then after that if you can afford the trick lightweight rotating assembly and valvetrain parts, then build and carefully balance your engine to rev high.
At a dragstrip, torque gets your car moving off the line, HP helps it go faster at the big end of the track. ET = torque. MPH = HP
You have a heavy car, so you want big cubes with Torque. Build for torque and the HP will come.
As far as motor mounts, you're going to have to change motor mounts for any of these engines, but that is no big deal. The 403 frane mounts should be the same as for Pontiac. The factory put Pontiac engines in these cars too. There are solutions for all the engines, even cadillac.