adding an extension should not affect the final torque assuming the fastener is being tightened properly in the first place. the length of a wrench would affect the torque but being you're using a torque wrench that is not a consideration. a threaded fastener is supposed to act like a spring and the goal is to have the proper preload on the spring. most people probably don't understand the mechanics behind a threaded fastener. think of it as an inclined plane (ramp) wrapped around a cylinder. this is why for a given amount of torque a fine thread fastener will apply a greater amount of clamping than a coarse one, it equates to a longer ramp.

imho one of the most important aspects of proper torquing is to make the final tightening in one smooth continuous movement. if you can't do this and stop before the final value then try to hit it again the break away can be greater than the desired value and the fastener will appear to be properly torqued but will actually be below value. it is very critical to make the final pull in one smooth continuous movement. i also think it's important to not have the fastener too tight before the final so you get a nice smooth final pull.

use the proper torque specifications, a REAL torque wrench, make sure all threads are in good condition and clean, use the specified lubricant. there's a good bit of difference between using oil and moly. for fasteners such as connecting rods or mains the fastener should be tightened in a manner that duplicates the way they were tightened when the bore was honed.

when it comes to torquing don't just assume that tighter is better. as for steel shim gaskets i'm not sure just what they will withstand. is this an iron head on iron block?

personally i don't think bolts have any place whatsoever in the mains, heads or rocker gear of a high performance engine. studs give more effective clamping and prevent wear on the block and heads. every time you pull up on a bolt it wears the block or heads.