its a crap shoot trying to get stock replacement springs that will sit at exactly the same stock height. will probably require installing them to check height then removing them to cut for the height you want & reinstalling.
ive seen quite a few turbo t/a's that have been converted to 400/455's & they kept the original WS6 springs & teh car sat about the same as with the 301t. while the 301 engine is lighter than a traditional pontiac v8, once you factor in the weight of the turbo parts its not that much of a difference... especially if you use an alum intake on the 400. if the stock springs sit at a decent height, i would swap the motor & see how the car sits, you might like it.
** however, personally i would NOT swap the motor, the 301t can easily be made to run as fast or faster than a stock to mild built 400, & the car stays original **
