[DF454]

If it is the HT4100 or the HT4500 V8 the head gaskets are not a DIY job.
The head bolts are torq to yeld bolts and can not be reused.
Torqing the new bolts in must be done exactly as spec'ed to avoid stripping out the block.
If that happens you must have the correct kit to drill out the threads and insert the timeserts (heli coils).

Also you must not leave a head off for to long as the cylinder liners may try to lift and damage the seal at the base of the block (causing a coolant leak). GM recomends bolting the heads back on (with the old bolts) or using deck plates if the head(s) need to be off the block overhight.

If the car is in good overal shape I would purchase a rebulit engine and swap out the original.