how hard would it be to replace the 4 cyl with the 305 out of my wrecked Camaro? I already know that the struts need changing, the tranny and shaft will be coming from the camaro. would I still be able to use the 85 firebird`s rear axle with the stock 3.73 gears or would it be best to use the rearend from the camaro?

 

The struts are the same, the springs are different. Motor mounts are different. But, you should be able to see what's different between the 4 cyl and donor cars and just change all that stuff out.

The 3.73 gears would be awesome, but it's probably an open rear, so "one tire fryer" and "one legged wonder" are terms you should get used to. Or, get a posi or PowerTrax unit for it.