Lower Control Arm Pivot Bushing

So... If the outer shell size is so critical, why does the aftermarket poly bushing suppliers tell you to slide their bushing into a 50 year old shell ? Also use the existing inner sleeve. Why not supply one that's compatable.
I'm not an expert but, wouldn't the poly bushing shrink or swell when the weather changed ? I always thought a tight hole was good. Poly bushings aren't tight, they slide right in.
I've done two rebuilds in the last couple years and I'm sticking with OEM.
I just did the complete front end of my 67 Barracuda. I rebuilt the LCA's with PST componets their poly graphite bushings and greaseable pivot shafts worked like a dream. The installation was simple you do not use the outer or inner shells the bushings press in by hand very snug and the shaft I had to lightly press into the bushing once in place you torgue the shaft nut to 145 lb/ft and your done. I also used the PST weld on LCA brace. Overall the car rides very nice and is alot tighter control.