Welcome to CHR!
As indicated above there are several questions that should be addressed before getting too far along the engine build path. Could you tell us a bit about the truck? If memory serves me correctly, the 3.08 rear end was used for the 305 with a three speed manual transmission in both Chevy and GMC trucks of that era. Is that the combo that is in the truck now? Do you have a budget? (Ha, Ha, Ha – right….. I just thought I’d ask.)

Starting at the rear end, if it is in fact a 3.08, I would recommend swapping out the gears for a one of the following (Assuming your 700-R4 transmission has the standard ratios, i.e., 1st - 3.06, 2nd - 1.625, 3rd - 1.00, and 4th [OD], - 0.69):
3.90 - for cruising and all around daily driving and occasional freeway
3.73 – if you’ll be driving on a lot of 55MPH plus roads
3.42 – if you be commuting at or above 60 or doing a lot of freeway driving

The stock converter in the 700-R4 is fine if you keep the build at or around 300 HP.

I would also concentrate on the 350 blocks. All things equal, if the one that has been bored .040 over is in good shape, use it. I would only use the other 350 if it is a 4 bolt main (or the bored one is bad). Completely disassemble the engine (don't forget to mark/label the rods and their respective caps) and have a competent machine shop hot tank and magna-flux the block. If it’s good, have them measure the block for being square. If it has never had the decks cut, it should measure 9.025”. If so, have them square it at 9.010”. This will give you a good base to build from. E-bay the other engines as cores.

Assuming the rods are good (they almost always are), have the machine shop clean them up and shot peen them.

Have your machine shop check the crank and only if necessary, regrind. In either case – buy new rod and main bearings from the machinist.

If the bores are in good shape without excessive taper, buy new .040 over cast flat-top pistons and a good quality set of rings from your machine shop and have them press the new pins into the pistons.

Have your machine shop install new soft plugs, all threaded plugs and new cam bearings. Buy a good double roller timing set from the machine shop as well.

Cam and lifters are a bit of a “personal taste” thing. I’m okay with about any brand but for anything except extreme builds, would recommend you invest the money (+/- $1,000) to convert to hydraulic roller lifters and cam. It’s a bit of work, but the safest way to build an engine that will continue to perform well over 100K miles. For street I like to use 110 degree lobe separation and stay at or below 270 degree advertised duration with lift no greater than .525”. If you use a bigger cam, it will begin to sound like a flatulating elephant and you’ll embarrass yourself in front of the rice rocket crowd (as well as have bad performance). Howards (and others) make complete kits that include new distributer gear and fuel pump rods. If your machinist isn’t familiar with such, call the tech desk at Summit and talk through what you’re doing and make sure you get all the bits and pieces. I like the cast aluminum front cover when installing a button on the cam (to keep it from “walking”).

I’m assuming you have 3973487, or 3973487X. This 487 casting was found on the ‘71-‘72 350 LT-1 engines and has 1.94/1.50 valves, 76CC combustion chambers and accessory holes. The 487X has 2.02/1.60 valves. The 487 heads will flow as well as the old factory “Camel Hump” head. The big problem is they are open chamber have poor combustion and burn characteristics compared to the earlier closed chamber style heads (or any modern after market head). If you have the 487X heads, 2.02/1.64 valves, they would be worth considering after a cleanup and magna-flux. Note that these are some of the last heavy castings and are not as prone to cracks. The 487X also got screw-in studs and guide plates. The reason I recommended cutting the block to 9.010” is it will help a bit with final compression ratio. Bear in mind that with a roller cam conversion, you would need to install new valve springs and use different push rods. Use new head bolts.

3947040 - They are 64cc with 1.94/1.50 valves but do not have accessory holes. E-bay them for cores - ($50.00 plus shipping).

The 441 head was probably the most popular of the 76cc castings. It was used on a ton of '69-'70 250/350HP 327 engines. Not a bad head, but it has 1.94/1.50 valves and will never flow as well as the 487 casting due to the small 155cc port volume. It does creates great torque and excellent drivability (think trucks, big Impalas, and Caprices), but wheezes after 5,000RPM. E-bay these as well.

Okay – after all that, here’s a couple other things to consider:

Aftermarket heads – aluminum heads are reasonable and already setup for roller lifters/camshafts. Again, a call to the tech line at Summit or Speedway telling them what you’re doing would help you get a good set of heads that are ready to bolt on and go.

Pullout engine/transmission from a late model vehicle - Done correctly, this is a drop in that can be very satisfying and brings your technology forward 30 plus years.

Final note/thought - buy what you can from your local machine/speed shop. We need to honor these men and women by using their services and they are good people to go to when a problem arises or you have a question. There is great value in a set of grease stained hands that resemble a pair of well worn leather gloves.

Probably too much information – others will chime in I’m sure

Best,
Glenn