Idle screws are on all 6 bbls, front & rear are in the base , front two you say are siezed up .so that is part of the problem .
what is the idle vacuum in gear & power valve rating .
often the air bleeds are a problem & inserting a small wire into the outer idle bleeds really helps as you...
The 74 usually had a carbon canister for emissions & the shroud probably made clearance for that so the 73 will fit but you probably need to remove the carbon canister .
The way I look at most things is it is not working now so how much worse can it be if i take it apart & try to repair it , if it still doesn't work oh well ...