The normal swap is to use the earlier alloy inlet manifold. This is fairly straight forward but the 4 branch and the alloy one don't match up at the fixing studs. You can machine to match or use stepped washers. Frankly the standard GT combined manifold is good and with a slightly larger exhaust system and some very mild engine work is good for about 145bhp. Up the cam to a D or a K if staying with Volvo. Good aftermarket ones are available, not the IPD/Isky ones. 123GT uses a C cam. Your 4 branch will need a suitable exhaust too but with a stock engine will show very little improvement for the cost involved.
All the above IMO. Did you get a 4 into 1 or a 4-2-1? The 4-1 tends to reduce low and torque but helps a fraction over 4k rpm. If you drive at those revs regularly you might just notice it.