I can't say from experience, and perhaps I shouldn't say anything for that reason, but I'm going to through my two cents in anyway, and if anyone disagrees please correct me.
If I were you I'd start with a 87'-92' roller cam block and use the stock lifters/spider retainer for considerable cost savings. The stock type lifters/"dogbones"/spider retainer for roller blocks can be purchased in a kit for almost 1/2 the cost of retro-roller lifters alone(used in older blocks).
As far as the heads you have(I have the same) I would disassemble them and do some pocket porting/chamber blending/polishing. Mine had sharp edges where the valve seat inserts were cut, and seats installed too deep IMO, that might have caused hot spots for detonation. Get two used valves from the machine shop you plan to use, and insert into the chamber that your working on to prevent damage to the seats before grinding. I used a dremel with a flexible wand, and sanding rolls as I didn't have an air compressor for a die grinder. I'm sure most wouldn't recomment a dremel, takes alot of time and not the proffesional way to do the job, but I think it worked ok for me.
I'd then get the machine shop to check the valve guides to see if there ok before getting a valve job done. After the valve job have them milled just enough for clean-up of the deck for gasket sealing. Then cc the chambers and equalize them for consistant compresssion and CR calculation(mine measured an average of 72cc +/-.3 after all the work).
I'd also get some beehive springs/retainers/seat inserts to work with a roller cam. I used Comp Cams behives pn 26986-16 springs, pn 795-16
10*retainers, and pn 4694 spring cups, and all fit withouut maching the heads.
This is probably alot of over kill for a 400hp, low compression engine, but I would at least do the valve job, guides checked, springs checked, deck shaved, and cc'ed. Use the springs recommended by the manufacturer of the cam you use. The springs on mine were not spec'ed for a roller cam. Being that your talking about a low compression motor smoothing the chambers may not matter, but it wouldn't hurt and may be whats needed to equalize the chamber volumes. I can't guarantee it, but I'm bettting mine will support a 383 making 425-450hp. I'll have to wait and see if I lose the bet.
I'd also use a Scat crank and rods, for a six inch rod as you can get it internally balanced(with exception of flexplate if using a one piece rear main seal used in roller blocks). I don't like an unbalanced dampner on the crank snout as the front main bearing has enough of a load as it is with the accessory drives(alternator/pwr steering/ AC). I have heard some negative rumors lately about Scat cranks, but I bought mine directly from Scat awhile back, and it checked out good with my mics. I also got the Pro comp rods that have the ARP 7/16"cap screws that didn't require any clearancing on the block. I haven't checked cam clearance yet.
For lower compression I'd use a piston with a 12cc "d-cup" and a zero decked block with a head gasket of 0.039" thickness(Felpro pn 1010 has a preflattened copper ring for alum. heads) for a 0.039 squish/quench distance to ward off detonation. I can't guarantee that it would be able to run using 87 octane, but with 72cc chambers it comes to about 9.5:1 static CR, and with a Comp 270 roller cam the dynamic CR comes to about 7.8:1. According to Comp's camquest software it comes to 437hp and 480ftlbs torque.
Ignore the groves in the chamber pics. I'm using a flat top with 5cc valve reliefs and the CR comes to 10.2:1, so I'm trying to get every bit of detonation resistance I can. Don't know if they work anyways.
Just some things to think about!