Looks like it should have either a WK or WR code engine, with #18 heads.
One site says the WK code was for a 4-speed, and the WR is just listed as manual. So, I don't know if that means a 3-speed came with the WR code block, or not. I'm sure Pinion head can give us the correct details.
You can look down by the passenger side of the timing cover and see if the last 6 digits, stamped on the block, match the last 6 of the dash VIN. Original engines will match. But this number is usually very hard to read, and covered with paint and grime.
The Wallace site shows that the carb number should be 7028269.
Also shows that a WR code engine had the 066 cam, and a WK code had the 067 cam.
Wallace Racing's Pontiac Engine Search
The #18 heads are said to have 1.96/1.66 valves, press-in studs, and 68cc chambers producing 10.5:1 advertised compression.
I've also read that a '68 should have an 8-bolt water pump, but the later size balancer, which is slightly larger than the pre-'68's.
"...From ‘64-‘67 Pontiac used a small diameter (5.25”) six bolt two-piece harmonic damper and from 68-79 a large diameter (6.75”), one piece four bolt damper was used..."
Harmonic Damper Replacement - High Performance Pontiac Magazine
There were a few exceptions, in the later '70's. Some of the low performance engines did not have a damper, only a solid hub to bolt the pulley to.
" The stock balancers from '68-'75 are all "the same", with the exception of the Ram Air IV engine. It has a slightly larger one. Very rare. The "performance" 400s from '76-'79 also got them. The 265/301 balancer LOOKS like them, but has "added" weight for external balance as is not interchangeable.
The "hub" (instead of a balancer) was used on '76-'79 "low performance" engines, such as those found in big cars, or on 350s in Ventura/Phoenix. It is actually the iner portion of the balancer. I believe the useage was to reduce cost on engines not needing 5,500 RPM..."
All this is probably WAY TMI. But, I won't charge ya'll any extra for it.