Torsion bar size??? 74 Swinger

Hey guys,
I need some advice on what size torsion bars to get.. Here's my set up: Stock Gabriel shocks front and back : new General Spring 5 leaf springs in back along with rubber bushings : Going with all rubber bushings up front with a complete rebuild of the front suspension : car has a slant 6 but within the next year or two I am going to drop a 360 in it. SOOOO. Should I leave the stock torsion bars alone until I put the 360 in? OR can I install them now with the current six? The eventual build of the car is going to be a nice street/ cruiser car with maybe a run or two down the 1/4 mile here and there