5m-ge future engine mods?

[CanadianBak'inSupra]

alum heads on cast iron blocks do weird things.
not 100% sure but at higher rpms the cylinders may be under greater pressure, forcing exhaust gases into your coolant, once it initially happens its going to go the other way and coolant will go into your cylinders.

get a compression check like you planned.
if you do a tear down, give the cylinder head a fine inspection for cracks, what may look like a casting turd/line could be a fatal crack. then go from there...
if it turns out your head is cracked, i do have 3 in my garage... and last i checked lordco can still get rebuilt heads for about 500$