The good thing about trucks is they generally have plenty of room for a fat tire, so even if they are lighter in the back, traction is possible. I would concnetrate on making the suspension work first, then the engine and torque converter combo. What sort of rear suspension does the 3100 have? Is is leaf spring or was that during the time with the long control arms and coil springs? Another good questions is what is your budget? And, as others have said, is this a dual purpose truck, street and strip or a dedicted bracket racer? Will you drive it to the track or tow it?

Give us a little more info on your plans and I am sure the folks here can help you come up with something that will meet your goals. I love those old round fender trucks!

Pat